Fuel Line Help

[1981Stepside]

Hi Guys - hoping for some help here - I've been having a weird problem with my fuel system and I think I've got it narrowed down, but need some confirmation. I've swapped a late model LT1 into my '81 stepside. The problem I've been having is when I tighten the fuel cap all the way it causes fuel to pour out of the bottom of my evap canister under the hood.
It originally had the 3 port sending unit using 3/8" for supply, 5/16" for vent and 1/4" for return. When I did the swap, I replaced the tanks with 1987 V10 baffled tanks and sending units and used LT1 in-tank fuel pumps. It is still a dual tank setup using a 6 port pollack valve. The vent lines are run directly from the tanks and teed up to the evap canister. I've searched the site and I've found some conflicting info - When square bodies changed to TBI in 1987, were the line sizes changed? I just connected all the lines straight as they were, but now I think the return line is crossed with the vent line. What size is the vent line on a TBI setup - 5/16" or 1/4"?