I am actually doing this conversion right now on my '66 Coronet.

Call around to your local salvage yards and ask for a tranny out of a 92-94 2WD Ram Truck, early to mid 90's full size Ram Van (150, 250 or 350) or the early 90's Dakota's had them as well. You want the A518/46RH. Most the time you can just reference the 46RH and they can find it from there

If you find a 4WD tranny with the transfer case attached you can buy a 2WD tail shaft off the bay for around $65 buxs. You will need to cut the bell off and buy a bolt on Ultra Bell housing from JW Racing, depending on what motor you plan on mating it to. I am putting mine behind a 383 and the part number for that is #3100.

There are a couple good "How To" threads here on the site that will walk you through everything from A to Z.

Good luck!