Mike, If you want to keep the flatty that is fine.I would opt for the T5 ,then you can change the rear and have open drive.The T5 will fit easily but you still will need to modify the saddle.I had a 36 5W that I kept the flatty but went with a T5 and 57 wagon 9". Them T5's IMO much better than the 39 type trans.Full synchro to.Ford trucks in mid 80's had a T170 which is a 4 sp.The 4th gear is an overdrive Plus its a top loader.Nice trans.