What size truck is it? 1/2, 3/4 or 1 ton? That will make a difference as to what can be done and possibly what gears are actually in the rearend. Generally speaking, a '61 has 3.90 in 1/2 tons, 4.56 in 3/4 tons, and 5.13 in 1 tons. Thae only avaiable aftermarket gears for that year truck are 3.38 for the 1/2 tons.
