Usually, as they've said, the JDM engines are just pulled from cars they took off the road because emissions are too tight over there. So they pull them, part them, and ship containers of SR20DET's or whatever over to us.
My father in law has a set of AMP brands on his F-250, they get down before my mother in law has the door fully open. I just trip over the stupid things lol.
They can be done quite economically, the trap most people fall into is they are coming from Gen 1 engines that are much older and were made to less exacting standards to begin with, so they start down the "while you're at it" rabbit hole.
While you're at it, may as well regasket it, and...
A 38 is about the threshold where thing's start breaking with power or wheeling.
If you have easy access to a 60, I'd use it. But for a driver, hunting, fishing truck that's not going to see a front locker and serious, high traction wheeling, I'd say a 44 is fine.
Tables add and subtract fuel and timing based on ECT.
Using a 1998 Camaro coollant temp sender is the fix, it has an electric output for a gauge, and one for the ECM.
I believe a simple remapping will be sufficient, without an wideband and logging, there's no telling. Unfortunately, the TBI systems are pretty primitive.