Cap the return line for sure, the venting will be taken car of by the lines coming to the charcoal cannister (or where it used to be anyway if you've deleted it). DO NOT drill a hole in the gascap, that's asking for trouble.
Get a fuel pressure regulator, 4-8 psi is too much pressure. The pressure could be overwhelming the needles/floats and flooding the bowls. I'm running 3-4.5psi from a Facet pump, and it's perfect. The needles control how much fuel enters the bowls as the floats drop, if they allow too much in at a time, you could flood the bowls too quickly, same goes for having too much fuel pressure. Have the floats been set to the proper height? this could also be a problem. Best thing to do is to make everything proper, and if you still have a problem, then start troubleshooting.
-fuel pressure regulator, set to 3.5psi
-set float level properly
-find out what the idles are exactly, they'll be named to this format: 65F2, etc. 65 seems WAAAAAAAAAAY big, considering they're probably universal webers and not emissions ones. Sounds like someone was trying to tune away a flat spot and just used a huge idle jet to compensate for something. You'll cruise on the idles, not the mains, so they seem way big. I'm running 60s on my Dellortos, and they are emissions models, which means I have to use huge idle jets. Right now I'm running only one sidedraft carb, and my idles are 62.5s.
-double check to make sure the cold start choke mechanism isn't leaking. this could also be cause of the fuel issues. It's a very crude device. Best to blank it off with some plates and gasket material.