LA360 initial timing too high - are there different timing covers?

When you say backfirinng, do you mean back through the carb? Or popping out of the exhaust pipes? Backfiring up the carb is typcially the timing being whacked out.

Be sure you look at the spark rotor position versus TDC and the distributro cap towers. That is rotor phasing. If it is out of whack, then it will crossfire to the wrong post on the cap; that issue can certainly change with timing adjustments. But it would not explain the engine being smoothest at 50 degrees advance.

What is the history of this engine, and what else can you tell folks about it?