What should 273 compression be

If that's a '67 engine in there, it has mechanical lifters. '68 and later had hydraulics. Check the valve lash, particularly on the exhaust valves. Backfiring occurs when the plug fires with fuel vapor in the cylinder and a valve is open. If the backfire is through the tail pipe, it's an exhaust valve. If it's through the carburetor, it's an intake valve. This assumes the distributor is set up properly.

On a warm engine, the valve lash should be .013 (intake) and .021 (exhaust). As things wear, it's not uncommon to have to re-set the valve lash. However, if the problem has been getting worse, it could mean that the adjustment screw on the rocker is stripped and not holding.