Boiling gas in 66 k10

[cwolfe]

Hey all, thanks to everyone for posting on the forum, it has saved me more times than I can count, but I can't find an answer for this one. I recently had to replace my exhaust manifold on my 250 chevy K10. The old one had a heat riser that was frozen in the position to direct exhaust away from the intake. The new one I bought has no heat riser valve, so exhaust goes right up to intake manifold. Now when it gets warmed up and I kill it, gas boils in the carb, and it is hard to start. The only fix I can think of is a carb spacer, but I can't seem to find one for my one barrel carb. Anyone know where I can find one. Is this even a reasonable fix? Is there anything else I can do to cool off the carb and/or the intake manifold. Thanks again