Really bad stumbling on the highway (400 small block)

[D-Day]

I can't figure for the life of me what is causing this problem. I took a trip to Florence, KY last night to drop my girlfriend off (about a 186 mile drive), and in the last 30 miles the stumbling started, and within the last 5 it was damn hideous going up the hills, whether the 700R4 was locked up or not.

Just before the trip I topped off the fluids, replaced a vacuum fitting T nipple, and made sure the choke cable wasn't sticking. Vacuum throughout everything was at normal.

I'm pretty sure it's fuel delivery related. It's either the carb, or the Holley electric fuel pump. I'm gonna replace both fuel filters for lack of anything better to do, take a look at the air cleaner...even if the air filter is dirty, it shouldn't have caused such chugging and lurching that severe.

It genuinely felt like I was about to run out of gas, or - when you forget to push the choke back in before accelerating hard, how it stumbles before you push it back in. However, it wasn't constant. On the way home, I ran for 40-50 miles like everything was fine.

Another concern is the gas cap I have. It's usually sort of loose, but I noticed when I filled up on the way down and back it was tight as hell, sometimes hearing a "pssht" of pressure being let off when I opened it. Thinking it was creating a vacuum in the tank, I let the gas cap hang off for the last mile home, and it still did it.

Right now my agenda is to pull the plugs, check the timing, replace the fuel filters...as for that, I'm not sure what else to check. Anyone have suggestions?

Its intermittent nature makes this hard to diagnose. Would something really bad like a cracked head letting coolant into the engine cause this? Thing of it is, it starts and runs fine at idle and lower RPM's...the stumbling is primarily noticed when the pedal is pushed down, when the need for more gas is called upon.