An overdrive GM NP540 is a good option if you can find one, and if it has the smaller input retainer (later ones have the larger input retainer that won't fit your bellhousing). I have two of the GM units, but neither are overdrive and one has the smaller bearing retainer and one the larger.
I put an NV4500 in mine using the stock bellhousing and clutch setup. I had to get a plate made by a machinist, a dodge gas input shaft and front bearing retainer, and that was about it. I'll link to my project below.
http://67-72chevytrucks.com/vboard/showthread.php?t=613597A Ranger Overdrive would be a very good option if you can find one.
