ok, you have the spicer 45 rearend. The available ratios for that rear are 3.08 or 3.92. The 3 speed trucks came with 3.08 rear gears.
The T5 transmission will bolt up without too much trouble, but 5th gear will be useless. Craigslist and ebay are your friends.
You should be able to get a good used transmission for less than $300.
