need help planning a fuel system

[RatMalibu]

here's the deal, currently I have a estimated 550hp (it put down 430hp to the rear wheels with a hurt automatic trans and a different carb with a 10:1 AFR lol) carbed BBC in my Malibu. I'm running a stock tank and sender with 3/8" aluminum line to a holley mechanical "race" pump then to a regulator to cut it to 7psi for my 750dp. Future plans are to convert to EFI so when my original tank developed a crack and started leaking I picked up a baffled Grand National tank for it and just used the stock non EFI pickup in the EFI tank. I have the GN EFI pickup and my original plan was to stick a GM TBI fuel pump in it and use it with the carb for now and swap to a EFI pump during the change over. I'm already planning to replace the Summit aluminum line with something that will handle the higher PSI, but I need help with the sizing. From my browsing the net some say 3/8" (or 06AN) is plenty and some say I should use 1/2" (or 08AN) and for return line some say same as feed line size and others 1 size smaller......I'm confused

I've been planning a return style regulator, anyone know of one that I can adjust down to carb pressures then raise to EFI pressure later?

Also the GN pump hanger I have has a 3/8" pressure line that is necked down to 1/4" because of the o-ring crimp on the end, a 1/4" return line and a 5/16" vapor line.....so I'm guessing I'll need to modify the pump hanger dependant on fuel line sizes. I prefer to keep it an in-tank pump....will a 255lph be enough? since those are the common upgrades for GN. Or should I be looking for a dual pump hanger?

Any help would be greatly appreciated