Stalling/Sputtering after 20 minutes

[blkseetin]

Need some help on what to start looking at.

I have a 70 C10 and it was running fine for a while. It's got a new carb (quad) and new HEI ignition. This problem seems to only happen after running fine for about 20 minutes. At about half throttle, it falls on it's face and dies. Sometimes I can pump it out of it and keep it running. The think I don't understand, is it runs great on my way to work for instance (20 minute drive) but on the way home after about 20 minutes it starts. Next morning it's no problems. The truck is pretty much factory and it has 50k miles on it. Oh, I was also smelling gas, and noticed it was leaking out of the gas cap so I got a new one. thats when this started happening. I was thinking vapor lock, so I started reaching back and opening up the cap on my way home, and that seemed to work at first, but the longer I drove, the more is start hesitating on acceleration and then even with the cap completely off, it was still doing it. My grandfather, also added a second and third gas tank (he liked to travel) years ago, and he has an inline filter in the fuel line. I've heard that unnecessary and was planning to try and remove that. If that doesn't fix it, I was going to replace the fuel pump. I've read there may be a vent line in the fuel tank that could be clogged, any advice on where I can find that?

Is this a familiar problem to anyone that might point me in the right direction?