I would check the gear ratio that is in the truck now. Even if it didn't come factory in that truck, if it came from a truck with factory trailing arms, it probably has 3.73 or 4.11. I read somewhere that the car and truck 12 bolts are completely different internally and the gears will not swap. My suggestion is to first check the width of the rear end. The original rear end for our trucks were 56" from backing plate to backing plate. Later truck rearends were widened by 2" but will still work in our trucks. My truck had a 12 bolt car rearend that a previous owner had installed... badly. Also, unless your rearend comes from a 70-76 Impala, its going to use a smaller bolt pattern than a truck. If your rearend is in that range, you can use it, but it will require some mods. First you need to cut off all the brackets currently on it. Then you need to make new brackets for the trailing arms and weld them to your rearend. Then you will need to make a new panhard bar bracket. One can be fabed to bolt on with the differential cover. We used the rearend from an 80 chevrolet truck this way. I have some pics I can get for you if you'd like.
