A carbureted engine usually doesn't usually need the biggest, baddest EFI pump. Even a little pump will flow a lot of volume at 10 psi outlet. Walbro has flow curves. 1/2" lines sounds like overkill. I currently have a Walbro (190 lph?) on my 65 Dart w/ a carb. I have a 3/8" supply and 5/16" return. With no bypass regulator, it runs ~10 psi on the return. That is too much for a carb needle to handle, so I use an in-line Holley regulator (pressure-reducing, came w/ a blue pump) to reduce to ~5 psi at the carb. The EFI pump is for future FI.
I expect the 10 psi return is due to returning thru the 1/4" vent tube in the sender I used. Whatever your design, you might need to install some pressure gages, at least temporary, to tell what is going on.