Good compression would be 100+ lbs per cylinder on that engine. Solid, even compression on all 6 holes is a good sign. That means the head gasket seal rings are intact and probably that the heads are not warped. The nylon pressure relief valves in each thermostat housing will get heat deformed. Replace them along with new thermostats. Resolve the original reason for the overheat (faulty impeller?) and start the engine back up. Let it idle and keep a laser temp gun aimed at the heads for 1/2 hour to see what happens. Backfiring through the carbs is usually related to bad timing, such as a sheared flywheel key, or a coil leads mis-routed to two spark plugs. If the engine suffered a high rpm overheat lockup, the key might have sheared. Unlikely, but anything is possible.
