Yes a back-fire can take out the power valve. Is the engine "stock" or modified? For modified motors, it is recommended you replace the metering plate in the outboard carbs to one that accept standard Holley jets so you can tune the end carbs for proper fuel mixture at WOT rather then richening the primaries.

You might want to check with ProMax Carbs (Moparts Sponsor) for your fuel bowl and or other parts for the six pak. They specialize in Mopar 6-paks, from rebuilds, complete stock restorations, to full race applications.

Scott