First off, don't buy a TNT unit new. The parts alone will cost your first born male child. Look on the auctions or craigslist and try to buy a decent used unit complete. Just be careful: These TNT units are always in the water and the parts corrode to the point where (although still working) they are impossible to get apart. It may take some time to find a decent unit--be very fussy.
People have come to me to repair their TNT units and after one look I refused because it was simply too much work.
BTW: Any Chrysler or Force TNT unit up until about 1994 will fit between your transom clamps. Any size, 3,4, or 5 cylinder will fit. (except the Force 70 and 75).
If you must buy one without the wiring harness, be aware that again, you don't need to buy new. You can go into a U-Pull-It junkyard and get all the components necessary from GM cars. You just need to be willing to do some wiring changes to the relay seats and we can guide you right here on the forum. It is not difficult.