Difference in 3 speed manual transmissions Truck/car ?????

Well you need to identify it. First what year is it. Crawl under there and take a couple of shots. There is only so many manufacturers PERIOD!!! of gearboxes through the years. Names such as Muncie, New Process, Borg Warner, Spicer, Eaton, etc etc. The list, really is not that long, and you won't find things like Eaton under a 1/2 T pickup

I can just about guarantee you, though, that a COLUMN SHIFT gearbox out of a pickup will have a longer tailshaft than an A body