Pins on the old ECM look fine, but a new ECM fixed the Code 51 and it's back to running like it was.

I haven't run into the sudden stall again, but only took a quick spin once I got stuff changed around.

One thing that is super obvious that I forgot to ask about is shuddering when decelerating. I have to disengage the clutch when slowing down for a stop sign, etc. because it lurches. But it starts lurching at like 2000rpm and it never did that before this cam swap and new injectors. I'll search it too.