85 Suburban w/5.3, leaking fuel at tank problem

[SBTork]

I just recently completed swapping a 5.3/4L60E from an '05 Avalanche into my '85 Suburban. I am having an issue with fuel leaking and tank pressure. I'm using the stock '05 returnless fuel rail at the engine, an '87 Suburban baffled fuel tank w/vortec fuel pump and a corvette fuel filter/regulator. The issue I am having is a slight fuel leak ontop of the tank. Also, when I went to put gas in today I noticed a LOT of pressure built up in the system behind the fuel cap. First time this has happened. I do not know where I should check first or what it could possibly be. My initial guess is that the tank is not vented properly. When I put things together I just hooked up the pressure and return lines. I didn't do anything with venting the tank or running a vent line from the sender. That may be the problem, I don't know? Any input from fellow LS swappers?