Been learning more on this EFI stuff.
On the return line, you might be able to use stock 5/16" line, with 3/8" supply, but if starting from scratch, 3/8" return is the easy choice.
It seems the return size will depend on the pump flow rate, and pressure.
The interesting part is the lower the rail pressure (by-passing more fuel), the larger return line needed. Also, higher engine vacuum will lower the rail pressure (regulator referenced to manifold pressure) even more from the static regulator setting. It just seems strange to me that the lower performance engine would need the larger return lines, but you have to by-pass more fuel.
I tested this idea on my setup. 3/8" supply and 5/16" return through re-pop steel lines (most of the way... some AN at connections). With the fuel pump "on", a really big A1000 pump and a charger on the battery to simulate running voltage conditions I could regulate a steady pressure down to about 30 PSI. Since I was running 45 PSI fuel pressure this works fine.
As mentioned above low fuel pressure would require a bigger return line. With 45 psi target pressure, 5/16" return worked fine. Also no regulation issues after quite a few miles.
