Race car or street car? On a race car you can simplify the plumbing by putting the regulator in the trunk next to the fuel cell and then you only have to run a single high pressure line up to the engine.
I would agree with mounting the regulator in the back and a short return line to the fuel tank. Most returnless OEM systems that use a regulator, like GM LS engines use the fuel filter with a return line as the regulator. By putting the return line close to the tank you do not transfer so much heat to the fuel by returning from the hot engine compartment. OEM went to returnless systems due to evaporative emission concerns, hot fuel likes to evaporate. Most OEM's now use the computer to vary the speed of the fuel pump to regulate fuel pressure.
