this is why spigot carbs are paradise. most fresh gasket sealant even the good stuff by permatex, rtv silicone and elastomeric rubber (right stuff) will fail from gasoline exposure after a few hundred miles. rtv quicker and right stuff lasting a lot longer.
if i was in your situation i would try to find the appropriate intake for the clamp carb or trade for a spigot carb. trying to match a header to a clamp phbg is a nightmare
just remembered amanda bandit got sent a clamp carb by mistake once and we turned it into a spigot style carb so she could mount it to her header intake
you just need to find an old long socket ~17mm and cut the hollow section off and jam it into the clamp. usually they are well machined around so you never have to worry about an airleak. then just go to autozone and get radiator hose and clamp it down with hose clamps. solid solid solid solution.
good luck dude. airleak troubleshooting sucks.