The Porsche 996, 997, Boxster, and Cayman are water cooled and have radiators in the front of the car. They also have tubing going to the back of the car into the engine bay. This makes swapping in a V8 a lot easier for us and the stock radiators are sufficient to cool an LS3 in most applications. If you are concerned about cooling capacity, you can add a center radiator, like Porsche 997 Center Radiator Kit. You can also get upgraded radiators like, CSF 7044 for 996/986 right and left, and CSF 7057 for center. For a 997/987, use CSF 7047, CSF 7048, and CSF 7049. For a Turbo/GT2, use CSF 7056, CSF 7055, and CSF 7053.
Some people have used electric water pumps, but they do not flow as well at high RPM. They often only flow at one speed, so they are good for drag racing since they will cool very well at idle. For most applications, I would recommend using a stock GM water pump. Stuttgart Muscle engine mount kits do not require water pump spacers and are intended for Corvette belt spacing. You will want to make some modifications to allow the water pump to fit better.
Assuming you are using Corvette belt spacing, you will want to use the water pump shown above, ACDelco 251-728 GM Water Pump with Gaskets. It has the thinner pulley and the top radiator outlet on the right side.
You can also try, ACDelco 251-726 GM Water Pump with Gaskets or similar, which has the correct pulley, but the top outlet is on the left. You will need a longer hose since the outlet goes to the right of the car.
Trimming the top radiator outlet to the second bead will allow the radiator hose to fit closer to the engine and help it fit better in the car. You will also have to trim the hose to fit as far in as it can.
On the side are the heater hose fittings. The larger one is the inlet and the smaller is outlet. They will interfere with the chassis, so I recommend removing them, but you can cut clearance instead.
UPDATE: Skip the steps below about removing the hose fittings if you can cut enough clearance to use these molded hoses instead.
Alternate: Remove fittings and thread holes
The heater hose fittings are just pressed in, you can crush them with vise grips to get a firm hold on them. They will come out after some twisting and pulling.
Now you can install Edelbrock 8166 Heater Hose Fitting 90 Degree 1/2 in. NPT and 3/4 in. Barb in the 1/2" NPT hole or 3/4" Hose ID to 1/2" Male NPT MNPT 90 Degree Elbow Brass Fitting if you don't mind brass. Use PTFE Thread Seal Tape to make sure it doesn't leak and tighten it so that it is pointing up.
Install Brass Hose Fitting, 45 Degree Elbow, 5/8" Barb x 3/8" NPT in the 3/8" NPT hole. Wrap with Teflon tape and tighten so that it also faces up. They may hit each other and not be able to be tightened if you install the small fitting before the large one, or if you try to use two 90 degree fittings instead of a 45 degree one.
You can see above how little room there is for the the heater hoses. Making this modification will make installing the hoses much easier.
Here you can see the tubes coming from the front radiators. They are 1 5/8" OD, but they have a special end on them. You can try to reuse the rubber Porsche hoses that fit them, but I just cut these ends off and left straight tubes. Be very careful doing this. Put a rag in to keep metal chips from getting in the tubes and files the sharp edges smooth.
You can use HPS 1-1/2" - 1-5/8" ID, 3" Length, Silicone Reducer Hose to fit over the straight tube ends. I have not had problems with them leaking.
I used 48" Stainless Steel Radiator Flexible Coolant Water Hose Kit With Caps from here to the water pump. I would like to make pre-bent tubes, but that still needs to be developed. I haven't had problems with these flexible hoses, they just are not ideal.
Use CVR TSH5CL Thermostat Housing and set the angle up, so you can attach the radiator hose, as shown above. I recommend using 160 Degree Thermostat, like Mr. Gasket 6367, since the lower opening temp will help with cooling. You will have to take the o-ring off the thermostat to get it to fit, but the new housing has it's own o-ring that seals it instead. Use HPS Silicone 90 degree Elbow Reducer Hose, 1-3/8" & 1-1/2" ID on the thermostat housing and the output side. Attach to the flexible hosing with HPS Aluminum Joiner Tubing with Bead Roll, 3" Length, 1.5" OD.
Install HPS Silicone Heater Hose, 1' Length, 3/4" ID on the 90 degree 3/4" heater hose fitting. Cut to length, about 1/2" to an inch above the engine mounts. Insert Dixon 179-1212 Brass Hose Splicer Fitting, Tee, 3/4" into the top as shown. The HPS Silicone 45 degree Elbow Reducer Hose, 3/4" & 7/8" ID has the 7/8" side go to the bottom of the reservoir and the 3/4" side toward the engine mount, as shown below.
Attach the HPS Silicone 90 degree Elbow Reducer Hose, 3/4" & 7/8" ID to the other side of the tee and orient as shown below.
Install HPS Silicone 90 degree Elbow Reducer Hose, 5/8" & 7/8" ID on the 45 degree 5/8" heater fitting as shown.
It's easier to get an assortment of hose clamps than to try to get the exact amount of each size, for example Hose Clamps Assortment 304 Stainless 8-44mm Range 80 pieces. I really like Gates 32932 PowerGrip Heatshrink Hose Clamp for the connections under the car and anywhere you don't plan on taking apart often. Expensive, but they really seal the joint well.
I like using steam vents on all four ports the heads have. Most later LS engines have the back ports blocked off. This is fine most of the time, but with track use, the number 7 cylinder can get too hot sometimes wit it blocked off. If you want to use all four ports, I recommend a kit like JEGS 53550 LS Coolant Cross-Over Kit. If you want to only use the front ports, you can try to use the stock crossover tube, or replace it with a kit like LS Coolant Steam Crossover Hose Kit 551675. Either way, you want the 1/4" hose from the steam ports to go to Joyway 1/4 to 1/2 Brass Fitting Reducer then 1/2" hose to the top port of the coolant reservoir.
You can use Porsche, generic European, or even Dex-Cool coolant, as long as it is compatible with OAT (Organic Acid Technology) coolant. The only type to avoid is the green IAT (Inorganic Additive Technology) coolant. If you want to use the green stuff, you will have to absolutely certain that you have flushed all of the Porsche coolant from the system and Dex-Cool from the engine. It is much safer to use something else OAT compatible, so mixing with any old coolant isn't a concern.
When filling the system with coolant, after ensuring all your hoses are correct and the clamps are tight, the easiest method is using a device like OEMTOOLS 24444 Cooling System Refiller Kit and jacking the rear of the car higher than the front. You will need a compressor to use this tool though.
Otherwise, you can just fill the reservoir normally with a funnel. DO NOT LEAVE THE CAP OFF TO BURP THE SYSTEM. This method will not work, because of the way the radiators in the front are setup. You will just recirculate coolant in the engine until it overheats. The black thing behind the cap has a valve on the top. You put the metal tab in the up position and the valve will be held open to burp the system. This valve only lets air coming from the radiators out and lets the rest of the system pressurize. Run the car with the rear elevated until it warms up, then shut the engine off, let it cool, and top off the tank. You should be able to drive the car now, but leave the vent open the first few hundred miles and top off the coolant tank regularly.