I'd recheck the timing and check the valve clearance again. Passing exhaust valves are a classic for backfire. Cat converters don't last long with detonation and passing valves burn up so don't delay. And don't be in a hurry or you'll be doing your re-build again...soon... It took me two head rebuilds to figure out that ...

'89 P'up, 2.6 I-Tec, 488,000 miles and done... gone to the great beyond