Haven't put a second tranny on a 1/2 ton, but I have driven a lot of bigger trucks with them.
First, how far will this move the 4 speed aft? Your gearshift will move back with it, might require bending to clear the seat. With 3.38 gears, and an o.d., your highway rpm will be pretty low. I don't know what engine you have, but you will be below the power band of most sixes. I would first build without the splitter, and see if I liked it.
