I have found that often the exhaust holes for the crossover slot are somewhat eaten up by the exhaust and not round, so the plugs do not perform a true seal. This is why I use a tap with grease on it (when working on an assembled engine) to keep most of the filings from entering the intakes exhaust ports. I simply tap and install a pipe plug, then reinstall the carb with the afore mentioned correct gaskets. Others have also welded them shut, or blocked off the ports at the intake gaskets (when the intake was off or removed). It's your car, hence, your choice. JM2C Timing is yet another possible contributing heat factor.