engine harness for $$$

Is this a Hellcrate? Is it drive by wire? Does it have VVT? There are some major differences between certain years. pass. car or truck variant, etc. You might want to think about how involved you want to get with controlling it and whether you want to spend time learning how to build a system on your own (MS3), be able to do some tuning/customization using aftermarket software (Holley) or just "make it work" (OE).

I have not done a Gen III swap myself but have been using Holley EFI stuff for a few years now. I'd wager the Holley is probably the most "efficient" choice since it's the largest brand with the most support. They have a complete package deal for $1,900. Not cheap for sure. The EFI software that comes with it is good though, I use it with the Sniper. You can get pretty deep with tuning if you wish. Holley makes it all pretty user friendly and the graphical interfaces are nice. Here's a link to their whole kit and kaboodle. Holley Terminator 2013+ Gen III ECU

Megasquirt has a stand-alone harness - MS3 Gen III wiring. If you like "DIY" and want to save some money then this might be a good choice. If it was me though, I'd be inclined to pay more for something I didn't have to engineer myself. MS3 info

I'm not up on what the OE or Mopar harness is about so I can't comment on it. Would you know if the "unlocked" PCM can be modified? You'd think that would be kind of limiting factor if you're stuck with a base tune, especially if it's from a truck.

If you use the OE PCM presumably you'll have to figure out how to make it work with your electrical system. You'd likely have to do something similar with the Holley/MS3 but my guess is they lay that out for you.

If you go with the stick that would pretty much alleviate the need for a trans controller. I'd want a Tremec over an 833 with a GV myself.