Just being curious was the Q-jet off the truck a power enrichment type?
Also I have had fuel being drawn into the secondary ports by being jetted to lean on the main jets. Looked like a trickle of fuel. Like the engine was starving and vacuum pulling it thru.
You must have that Pontiac 350 built pretty strong running that big of a carb. I'm running a 780cfm Q-jet on a 455 Pontiac and that feeds it thirst for fuel great.
To be honest on the gaskets I really never paid much attention. I just always "matched up" what I took off.
Sounds like you will become the Q-jet wizard. Good Luck!