Intermittent Backfiring After New Head Gaskets

that gasket size discrepancy by itself is pushing 2cc

BTW
a coolant recovery bottle, in order to work as a recirculator, has to have a special rad cap and and a hose running down to the bottom of the recovery tank. The idea is that, as the engine cools down, and the coolant with it, the contraction of the coolant will pull the stuff from the expansion tank back in.

However, your 273 should NOT be running hot unless it has a cooling system failure. Advancing the timing is not the answer, but is only a temporary band-aid. Just maybe the head gaskets/warped heads were the cause, that remains to be seen.
If your overheat issues go away, then you can use the top 1inch or so of the top rad-tank as your recirculator, just as the factory did.