What numbers are stamped on the choke tower?
'63GT covered the three possible balls:
Accelerator pump weight under the shooter
Accelerator pump check valve
Vacuum secondary signal.
Kits often include extra parts that cover a different version of the carb. Holley calls the type of carb a 'model' and each version has its own engineering 'list number.' That, the revision number, and date of manufacture are on the choke horn. The metering blocks will also have similar numbers, but often they don't overlap.
Referencing to the backfire once again. Rich or lean, if there's a misfire, then unburned hydrocarbons will be available in the exhaust. It just will need enough heat and pressure to set it off in way that bangs. If I was there, I'd look at all 8 spark plugs for clues of whats been happening in the chambers. Just obvious stuff like carbon build up, electrode condition, gaps.
