My stock stroke mild 360 build ;)

[1] Backfiring on decel. Probably carb too rich at idle. [ It is a Holley, right? ]

High vac on decel pulls extra fuel from the idle cct. Rich mixture burns slower & with your low init timing, mixture is probably still burning when exh valves open...
Lean mixture also burns slower. Air leakage [ gasket failure ] into the headers/exh at the heads can also cause backfiring, but I think rich mixture more likely.

[2] High cranking pressure. Simplest, cheapest is to try is retarding the cam 6*.