The NP241OR fits with the Savvy skid with some minor tub smashing. No custom trans mount.
The Atlas in any iteration does NOT fit and will not work without some major cutting of most of the stuff that matters.
The 241 is a basic swap, not cheap, but since it was bolted into the rig from the factory albeit with a lowered skid, it is still a direct bolt in with the right parts. The only gotcha nowadays is the lack of a reliable switch for the 4wd light on the dash and the stupidly insane prices for a used 241.
I thought there was a GM sensor that took care of the 4wd light?