If you are going to do that, and if original does not matter, you'll gain more by going with a 5 speed. You'll need some more parts , but you'll get OD . The 3spd and the 4spd have the same 1:1 final ratio. JMO
As posted if you are going to pay for the 4 speed. You might want to consider the T-5. Depending on the money. If you do go to a T-5 your rear end will be a little low. You'll have to be doing 70 before you can really use 5th gear. The 3 speed cars had 2.80 rear gears. A 3.00 of 3.25 would be better. Going to 3.5 or 3.55 rears make 5th very usable but you really loose 1st gear. You'll role about 5 ft and have to shift to 2nd. Alot of 2.8 setups just don't use 1st gear.