In my experience backfiring is caused by 2 things.
A lean condition from off idle caused by to lean of a squirt from the acc pump or to small of a squirter.


I had that problem on my old 383 when I put a new 750 Holley on it. I cured it with a little acc pump arm adj and the next size squirters.

A cam with a bad exhaust lobe can cause it also. With the valve not opening all the way the hot charge doesnt get pushed out and when the new fuel/air charge hits it there is a backfire. The backfiring is constant tho ans increases as the RPM increases.

On the hard starting after warmed up Id say you have a heat soak problem. Try a thicker carb to manifold gasket or maybe a thin phenolic resin spacer. .