Top end 2.4 rebuild and engine reseal. Car backfires and now engine is leaking

[Sunroof]

I probably know the answers already, but I hope some of you chime in to offer opinions. Its along story, but I will make it short.

1973.5T with CIS has broken ring and engine get rebuilt with valve job, new nikisil pistons and cylinders. Bottom end is excellent. Engine resealed.

Shop replaces only one leaking injector (#5). The others sent to a injector shop for cleaning. Performance after rebuild indicates hesitation in the 2700-3500 rpm range. The remaining injectors are suspect although fuel pressures are right on the mark.

I take the car out for along ride (30 miles) and just before arriving home, as I accelerate from a stop, I get a MASSIVE backfire!! RPM goes to zero, however, car restarts on a roll and I drive home with no problem.

Next morning I find a small pool (maybe three thimbles full) of oil on the floor. I look under the car and it appears it might be coming form the front of the engine (toward the trunk) but not certain where its leaking from.

The pop-off valve saved the airbox, but what could have happened with such a loud backfire?

I think its a bad or several bad injectors or a leaking cold start valve.

Opinions please.

Thanks

Bob