RAM 518 OD will NOT fit your tunnel. But a 700R4 GM may if your leaning toward an auto. So will a Ford AOD. Both are designed from the ground up to be OD so their OD internals are "internal" not a bulky bolt on tailshaft rig.
Best to skip the 700R4 as you need a $1000 transmission computer to run it, and they are bigger. A 200R4 is a smaller bolt in, you can run a one 12v wire lock-up or not. It's a 4speed OD with a 2.76 to 1, 1st gear and a .67 to 1 overdrive, Pic #1. They can hold up to 1000 +hp when built right and be bolted on to small blocks, big blocks and the new hemi's. I bought the 750 hp/ 650 ft lbs tq lighter duty version.
You still have to cut the crossmember to make room for the driver's side tunnel crossmember, and reinforce the area. You also have to make a new rear crossmember, and the rear mount.
If you want to run a 518 Mopar trans, you need to basically remove the entire tunnel section of the crossmember and reinforce the area Pic #2. You'll also need to make your own rear mount, or you can buy a Fontain's crossmember already made and tilt you engine/trans way down to clear the crossmember. Pic #3. I don't know how you would ever get the pinion angle correct, but some people are running them like this.