Leak
I would suspect the other two allens at the rear of the block near the cam. If one was loose, the others may not have been sealed correctly too. Does it have oil restrictors? Would also check at the cam plug (freeze plug) and see if it is leaking there. My 377 would leak at the intake manifold where it meets the block near the distributor. The problem turned out to be the manifold was poorly machined at some time, would leak at the strangest times. Used gasket in a tube to seal it.