what is the best overdrive transmission for a Duster

Sorry forgot to mention its a 727 automatic. I am mainly searching around on Craigslist, there are a lot of wrecked dodge trucks and such that say the transmission is good but I don't know if what they have would fit my car.

Like what did the 200r4/a500 normally go in? If I find a good deal on a wrecked car I could pull it myself.

Haha and ya rumblefish360 I knew when I built the engine I wasn't going for mpg but 12-13mpg would make it a little nicer to drive lol So what can I do with my 727? is the 518 a whole new transmission instal or can I add the overdrive unit to the tails haft of my existing transmission? I am prepared to shorten drive shafts and do a little modding, including cutting holes and installing pedals if I find a good manual overdrive.