85 kjet exhaust backfire USUALLY means running rich. i trust the cpr is correct at the correct temps-50 psi sounds right for fully warm. no rest pressure just means long crank, usually not a running problem after starting. did you test fuel press while it happened?

if you unplug the o2, does it go away? is the freq. valve buzzing when o2 is plugged in? is it even, that is on-off-on-off, not onnnnnn-off-onnnnn-off or the other way around? have you driven it enough to notice a mileage difference between o2 plugged in or not? have you watched the o2 while driving?

injectors leaking? gotta pull them and check with fuel pump running. more than 100k? they're worn out.

i'd also look at the coil's reserve kv with an ignition scope. not enough spark looks like too much fuel. in this case, the coil's a long shot.

how's the engine harness? could be the o2 ecu isn't talking nice because of flaking insulation you can't see. luckily, it's not a bad rebuild. much fewer wires than an lh car.

out of ideas. good luck, chuck.