Weird LT1 issue

Some days on first start, engine occasionally would seem like it had flooded, like a stuck injector had drain the fuel line overnight. Holding the pedal to the floor would give the impression it was trying to catch. When I would let go of the key the engine would stop, then proceed to diesel backwards a few revolutions and some whitish vapour would appear from the grille where inlet was. Two weeks ago during the cough cough cranking stage, there was a very loud back fire and a bunch of white vapour came out from all around the hood. Engine started. Took it to my cousins shop and told them I figured I had an injector that was draining the rail. Well the backfire blew the valvecover apart and pushed the gasket out. None of the plugs were fouled, but I did put new ones in. They were 8 yrs old and 80,000km(50,000 miles) and had a gap about double the .050" spec. So far it hasn't done the hard start again, nor is it missing or misfiring while running.

Any Ideas?