I'm assuming that you mean a 4 speed automatic overdrive transmission. A 4 speed manual would not help you. There are several overdrive automatics to choose from. The 700R4 has a 3.06 first gear which wold be about useless with a 4.xx rear end gear ratio. The 200 $r would be a better choice because it has a normal first gear and a better overdrive ratio, plus it's the same length as the TH350 and the cross member doesn't have to be moved.
If you do go to an overdrive transmission you will need to get a good drive shaft. I would recommend at least a 3 1/2 inch tube to beat deflection at high drive shaft RPMs. Otherwise you are going to get a vibration somewhere around 60 MPH or higher.