You'll need 4 pieces of information to get it right
Tire diameter
rear end gear ratio
Number of teeth on the transmission speedometer gear
Number of teeth on the speedometer gear
Then you can use this to find the right combo of gears for the tranny and speedo cable
http://www.tciauto.com/tc/speedometer-gear-calculator
Most Saginaw came with an 8 or 7 tooth gear in the tranny, although that can vary.
How far off do you feel you are?
You can jack up the truck and rotate the rear wheels to get an idea of it's gears.
Pull the speedometer cable and bullet out of tranny and count the teeth on that gear. You might be able to guess what the tranny gear has based on that.
Remember, the variable you are off will change at speed. The faster you go the more offf you will be. The variable is in percentages. I have to get unlazy and pull my speedometer cable to change the gear out. Mine reads about 20% off with the tire and gear changes I did.