Alright, so i was talking to Fastlane about my fuel system problems in his build thread, and thought i would start a new discussion rather than hijack his thread any further. Here's the conversation.
The vapor valve is there, and brand new. the vapor line going to the engine bay is mostly new. All the seals on the tank are new. THE TANK ITSELF IS NEW! The only things that aren't new are the filler neck, which is not available new, and the carbon canister and purge solenoid/valve.
I could buy a new carbon canister and solenoid, but that's almost $100 and I've blown so damn much money on this so far with no result. And the solenoid does not throw a code.
That said, the tank vapor valve being new does not guarantee it works (my fuel pump roulette has taught me that), i suppose. But i don't really know how to test it. I tried blowing into it, but that didn't do anything. I know it's not the gas cap, since i have the symptom (gas spitting back at me) when the cap is off. I suppose one possibility is the air passage in the filler neck. I can't say with absolute certainty it's clear (although i don't see how i could have missed that).
I'm hesitant to go to the trouble of pulling the tank because of the possibility I'll put it back together again and it still won't be fixed. That would be a HUGE letdown.