1999 k2500 ccsb build! Few questions in here.

Hey all like title says I'm about to do a swap into my ccsb. Truck was originally a 5.7 crazy under powered. So plans are a 5.3 swap which I have a full donor vehicle to gather parts from.ive done a ton of reading and searching but most links are expired or there is newer and better ways to do something.

Truck has the 4l80e in it, I know the 5.3 will require the new flex plate with spacers. Will the flex plate off the 5.7 work on the 5.3? I've never seen it asked.

Motor mounts are another. There is so many companies out there selling them now. I would imagine just the plate to adapt to the factory clamshells is the easiest. However, what offset plates do I need to retain factory cross members? 1" back? 1" forward?

I plan on leaving factory computer in the truck so it retains abs and all that jazz. How do I wire up the check engine light to work with the 5.3 ecm instead of factory?

Most people strip their stand alone harness down I don't plan on doing that. With the extra wiring still there will it trip the check engine light if something isn't plugged in?

For the wiring contact Current Performance(just google there name). They did all of my wiring for me swap into a 98. Very helpful. I used one PCM and harness for everything in the truck.