The 3.07 rear end I have was taken from a 1969 C10 which had an auto transmission. Pretty common late 60s rear end if auto transmission was there. Good rear end, especially for a late model 235 if you mate it with a T5 with the gear chain beginning with 4.03 and ending with .86. Almost can't be better. With that setup at 60 I'll be at 2000 rpm...right at the max torque for the 1959 235.
As for rpm, if you had the original 3 speed (a 1 to 1 final ratio) and the 3.07 rear at 60 your rpm should be around 22~2300 if you have 27 inch tires.
If you have the 3.73 rear and 27 inch tires, your rpms will be around 2800.
One easy way to know what rear end you have (without counting teeth) is to look for the GM code stamped on it. Usually people don't change ring and pinion without good reason and with the 12 bolt GM truck rears you have ranges you must stay within if you do want to make changes. You have 3 deck heights: 2.73 and lower, 2.76 to 3.42 and 3.73 and higher. So you couldn't convert a 3.73 into a 3.07 or 3.08 rear end.
