Nice little laundry list of things to iron out. Regarding the body, would it make sense to go with a new repop and cut it specifically for the mid-motor, leaving as much rearward material as will fit? It might bring the body close enough to the rear bulkhead to run a flipped newer style wing mount as a body mount. I might be talking out of my ass though as I've never seen an uncut XX body and am unfamiliar with the contours and clearances of this buggy and body.
A "fresh" body has the rear section sloping down, so it would not meet the body post on the rear tower as the body would drop too low. So, I need to come up with something else.