i have had a backfire liket this once also, fuel in oil is probably why along with a non pvc setup. as the engine warmed up gas fumes accululated in the engine and a little backfire from the engine or something and wham. your engine compt is one lttile bomb.
mine blew one valve cover right off off a chev 305 and left the other hanging plus ballooned the pan never did find the dipstick .