Heat

The turbo(s) mustn't come near anything that will be affected by heat and there must be plenty of room around them for the plumbing as well (which gets hot too).. This applies to the compressor housings as well. The compressor housings may not run hot while the engine is running due to inlet air cooling them, but when you shut down heat will soak through the cartridge and make them almost as hot as the turbine housings, so take this into consideration. When an engine shuts down, there is little to no air movement under hood, so things can get hotter. Even an idling engine at least has the cooling fan causing some airflow under the hood.

Heat Retention

The power used to power a turbocharger is exhaust gas velocity. When temperature drops in an exhaust system, so does it's velocity. This is important when considering placement of a turbocharger because the farther the turbo is from the engine, the greater the temperature drop will be. In this respect, putting the turbos as close to the engine as practical will give best turbo performance. Sometimes is it not practical to have a turbo very close to the engine, in those cases, making the manifold (or header) from a material with a strong fatigue resistance will allow insulating of the piping.

Plumbing

A turbocharger requires quite a bit of plumbing. There is turbine inlet and outlet, compressor inlet and outlet, oil feed and drain, and sometimes coolant in and out. As with any type of plumbing, sharp bends cause a restriction, so it's best to put the turbo where the bending required will be least restrictive. Also, as with any type of plumbing, high pressure sides are less affected by restriction than low pressure sides. In other words is you have a choice of a 90 degree elbow at the compressor inlet or the compressor outlet. It will do less harm on the higher pressure outlet side. It is harder for a pump to pull than it is too push. With the oil system, A 90 degree fitting at the pressure side will harm very little since most engines oiling systems can supply more pressure than the turbo needs to begin with, but a restriction on the drain side can back up oil in the cartridge cause big problems. And for the turbine side, the exhaust gasses entering the turbine will (should) be much higher than in the exhaust system after the turbo. The idea here is not to try redesigning the chassis around a "correct" turbo plumbing system, the idea is to compromise the fewest of these things for the space you have to work with. This may require relocating some under hood components and/or protecting others. For me, I relocated my Duraspark box and some wiring. What I did, which may not be an acceptable option to some people, is I converted to manual brakes and steering. By removing the power booster and power steering pump, I gained a lot of room for a better turbo system.

Other

Once you find the best place to mount the turbo, consider the effect it will have when you do routine maintenance on the car. If you have a solid cam that requires periodic valve adjustments, it would be nice to be able to remove the valve cover(s) without removing a turbo. Spark plugs are another thing, make sure you can get to them without too much trouble. When you are mocking up a manifold, plugs and wires should be in place and checked for enough clearance. Many V8's have the dipstick on one side or the other, it would really suck if you couldn't check your oil. You can take a few pictures of the engine bay before you start taking it apart for quick reference as to what might be in the way. It's easy to forget the little things that make a big difference.