If you switch shifter to floor shift you'll need a floor hump where shifter comes through the floor. They are available repop, or you may find one on a parts car you can cut out. 4 speed seems to be the most desired standard trans, and it is a Mopar 833. They are available in overdrive also, and are routinely for sale on Ebay for a couple hundred. I also am pretty sure that you could reuse the original 3spd. Below is link to most info on the 833 4spd. Mike
http://www.slantsix.org/articles/4-speeds/ODA833fourspeed1.htm