Good question. Let me know when you find out. I am currently right there with my efi to 351 carb swap.
I have stock setup from gas tank to engine compartment, 3/8 hose to FPR, fuel pressure gauge then to carb and I am getting 10-11 psi with the adjustment screw completely removed from the FPR.
I need to get this down to 6 psi. I might try a larger fuel line for the return.
You can't get reliable pressure and delivery with an EFI pump and a carb setup.
Do it right, low pressure electric fuel pump, regulator. Mod the EFI fuel tank by swapping the in tank pump for a piece of hose with a garden sprayer pickup screen on the end. Use the red/green wire on the ignition coil to trigger a relay for the new electric fuel pump.
new electric fuel pump.
Even better, get a mechanical diaphram pump like on the 85's. No wiring or relays and the volume slowly rises with rpm, which is what you want. They are dead cold reliable and 've had the same pump in there 4 years now.
When I did the conversion, I tried every half assed trick....double regulators with return, variable resistor pack on the pump + wire. It either overflowed, over powered both the regulators then overflowed or didnt produce enough volume and sucked the bowls dry.
Jrichker is right, get the specialty low pressure stuff or you're looking for trouble.
If I went the mechanical route, would I still need a return line? I haven't done the research yet but this seems like a fools proof approach. I am planning on installing a fuel tank insert to replace the stock pickup. I'm just not sure about the return line.
I went efi to carb 5 years ago. Stock fuel pump with a mallory fuel pressure regulator, and havent had any problems on my holley 750dp. I didn't have to use 2 regulators and I just installed a gauge to monitor the fuel going to the carb. Also ran a return. http://store.summitracing.com/partd...4294880914+400249+4294891681+115&autoview=sku
With mechanical FP, there is no return line. A single hose running to the tank is all.
As for the pickup, you drop the tank, remove the EFI pump and replace it with fuel line just a bit longer than necessary to reach the bottom of the tank. This way the hose turns sideways a bit and acts as a scoop for the bottom of the tank. Try to drop the hose inside the baffle.
I cap the return line but dont remove it.
BTW, that is the EXACT mallory regulator I had plus an inline el-chepo holley non return regulator and with both on, it either lost volume or overflowed anyway.
Look at it this way: It takes a lot of energy to pump high pressure fuel up front. With the mallory solution, even if it works, you are wasting a lot of electricity to pump fuel back and forth between the regulator and the tank. Just do it right the first time.