Got a rebuild 91 with 393. I put a Holly Sniper EFI and an evil fuel pump in. Issue is it sucks fuel from the front tank & returns it to the rear tank & vice versa. Anyone know what the issue is? Why does one fuel tank suck fuel & return to the other fuel tank?

 

Because the one way valves in both pumps are shot
This was the fix back in the day
Now days you just replace fuel pumps over and over

 

So by replacing the fuel pumps with new ones, the new check valves come with the pumps

 

Right
The cheap 58 dollar China pumps fail quick
The 150- 200 dollar Delphi pumps are better but no match for a Motorcraft pump