It sort of depends on what the ratios in the Saginaw are. If you know how many lines were cut into the input shaft, we can tell you what the ratios are. As for 4th speed, it will work. You'll run about 25% fewer rpm at highway speed. Hint: 2 lines in the input shaft is a good situation...
The best of all worlds in my opinion is a T5 with a .86 fifth and the rest of the gears beginning with 4.03 (first). A 4.03 first will give you a starting line ratio of 12.37 and this will indubitably let you operate it like a pickup truck. Your original starting line ratio with the 4.11 rear end was 12.08. 12.37 is awfully close, so it will feel like you have the old 3 speed until you get to 4th and then rpms will drop 25% and when you get to 5th on a nice flat stretch with minimal wind, you'll drop them another 14%.
The original 3-speed is 1:1 in 3rd gear, the Saginaw 4-speed is also 1:1 in 4th gear and the T-5 is 1:1 in 4th gear, so where is the "25% fewer rpm" coming from?
If the OP installs the 3.07 rear he won't need an OD trans (T-5) as he'll never get out of 4th gear. The T-5 works well with the stock 3.90/4.10 rear or something in the high 3's.
Mike B
