[Mike C]

You won't need to change the rear if you add the overdrive. A TH200 4R has a .68 overdrive. If you are turning 3200 rpm on the road, that drops to 2176 which should be a sweet spot for a carburated motor. A TH 700R4 has a .70 overdrive which drops the 3200 down to 2240.

An OD trans with 3.07 gears or less is almost not useable because it lowers the revs so much. Especially true with a smaller motor and/or one that's not tuned well.

One of the primary reasons newer cars are more fuel efficient and driveable is the more transmission gears not the motor and FI. They help, but it's all about the transmission and final drive selection.