1974 Slant 6 backfiring through carb.?

Sounds like it jumped teeth on the timing gear.

When you hit the starter in the morning the crank turned and the cam did not want to. A really worn out chain will do that. It is like having a loose chain on a bicycle. It may cruise just fine, but pour on the power and the chain will jump on the srockets.

Could be worse..at least it runs!