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"?

[1981Stepside]

OK - so for anyone else who experiences this problem - I've confirmed on a new 3 port FI sending unit that the 5/16 line is used for the fuel return and the 1/4 is the vent. Strange that GM would switch the two lines when the transition was made from carb to FI but, there it is.
Now to drop both my fuel tanks to re-plumb.