what is the best overdrive transmission for a Duster

So the 1976 one won't require cutting the crossmember or any serious modifications?

It is a complete bolt in swap with all the right parts. If you have an automatic however, you will need a 4 speed floor hump. That does require cutting a portion of the transmission tunnel out and installing the 4 speed hump. Brewers sells fiberglass ones for 99 bucks and you can install them with screws and seam sealer. Other than that, it is a complete bolt in.