At first I thought it was the timing and I pulled the timing cover but the chain and distributor gear look fine
Not to be a smart-a$$, but did you check BOTH sides of the timing chain for tightness?
On another note, I have heard quite a few smallblock chevies "clapping" (backfiring) thru the carb, usually is a bad cam or burnt valves, but that doesn't come & go, once ya got it, ya got it.
