If you decide to use only the engine and trans (not cluster or other parts), you can get stand alone ECU's and wire harnesses. It's when other module controlled parts are used that it becomes very complicated. Every make is different but, some use the ABS for speed signals or other inputs and without the ABS, none of it will work (just an example). In some cases, the fuel pump is controlled by a module, which gets inputs from the ECM (and possibly ABS, BCM, etc) then controls the pulse width of the pump, which sends data back to the ECM...........................Well, you get the picture.
I am by no means an expert on retrofitting new to old, just know how integrated everything is on newer cars (I'm an auto tech). Example: Recently we had a 2016 Chevy Traverse in the shop that wouldn't crank, the cluster was dead, the power windows didn't work, among a few other things. Problem ended up being a shorted (due to salt water intrusion) side obstacle detection sensor/module in behind the right side of the rear bumper. Took the whole network down so everything on that network also went down and some things on another network were disabled due to missing inputs from modules on the network that was down. I love my job, I love my job, I love my job, lol.
There is a member with a late Hemi Mirada (or Cordoba?) on here. Apologies to him and you, I can't for the life of me remember his screen name at the moment.