I had some backfiring from my 283 prior to a complete new dual exhaust, now no backfiring on engine braking. Point being, do you have exhaust leaks near the engine, on deceleration it will suck oxygen into the exhaust leak and backfire.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy