Hi my friend, looks like your having fun, here's a few answers to your questions, use a mpi fuel tank because it has a return fitment in it, you don't need to modify rear subframe, the fuel tank should bolt straight in after your boot floor mods, fuel lines are 3/8" and 5/16" this way you can use AN fittings, you can use p clips every 300mm for fixing lines, interior lines have to be continuous from rear bulkhead to front bulkhead, use bulkhead fittings, the larget fuel pipe is return line, you will need high pressure fuel pump for the injectors to operate, I have posted a couple of videos of me installing fuel lines on my son's classic rally mini build, which is going to run injection to. Hope this all helps, cheers Marty
3/8” fuel line will supply more fuel than your engine will ever need. But more is better than not enough. 5/16” will also be adequate for about 150-180bhp@ 3 bar. Also you don’t need a return line although you can run one. To go without a return, You can run a dead head system so the pump and regulator and return all in the boot and just 1 feed pipe going up to the engine.
Yeah could do it that way would save running another line, I went with two lines because that's what was on the car originally, the larger 3/8 is the return back to the tank, my regulator is variable as running a turbo, so starts around 3 1/2 psi on idle and increases with boost, there is an extra pipe running from plenum to regulator. Cheers Marty