Well this is my first post and I guess I should introduce myself. I'm Dakota. Gm service tech for 7 yrs.

Anyway, my 96 5.7 spun #4 rod bearing, and I have a 5.3 laying in the shop that I would like to swap in. I've done quite a bit of research but still have a few questions that I hope you guys can help me with.
I'm not sure what year the engine is. But I believe it came from a van. It had a bracket bolted to the pass head that trucks and tahoes don't have, and the brake booster line had a plug instead of a nipple. It is drive by cable and it has an egr valve.
For a harness an ECM, what I understand is that I need a harness from a 99-02 dbc 5.3 gm vehicle and a 12200411 ECM. I plan to modify the harness myself. Do I keep my old black box? What do I need to keep out if my old harness to make all gauges work?
I plan to run ac. Can I run the express 2500 5.3 compressor? It seems to be small enough to fit in the factory ls location.
I'm using my 96 4l60e. It has been heavily upgraded. I know I need the 6.0/4l80 crank spacer. Which flex plate do I use, The 5.7 or 5.3?
I have the 1" setback mount adapters.