The advantage, as Caroline's Truck says, is higher HP.
Another advantage to a camaro w/c box with s-10 tail shaft that has the shifter box in the right spot, is the gear ratios. A truck tranny has lower 1st gear and o/d, the car tranny will have a higher 1st gear and o/d. For example, maybe 1st from a truck is 3.72 and from the car its 2.35, fifth of .78 in the truck and .62 in the car tranny.
I recently called a tranny shop to get my t-5 rebuilt and they quoted me $900. I'm gonna buy a rebuild kit for $125, some upgrade parts (rear bearing retainer), and spend the difference on something else I can't do myself.