65Val: I did consider the heat from the exhaust crossover. There is only a thin paper gasket between the carb and manifold. I'll try a spacer next. Do you remember what size worked for you? How did you block the crossover? Did you remove the intake and plug it somehow? My heat riser on the passenger exhaust manifold has a broken spring, so last year, I wired the counterweight in the open position and verified by looking into the manifold to see that it was open, and not wired closed.
Thanks for your help, I'll let you know how the insulator works out.