I didn't realize that the 1 1/2 ton trucks had a 36" frame. Moving the spring seats an inch on each side shouldn't be a problem. If you can squeeze the motor in, getting the rear to fit should be no problem. However, you didn't mention. Does the 70 series have air brakes. If it does, it would be a problem. Unless you go all out and put air disc on the steering axle, it wouldn't really work. However, the Eaton 3rd member will fit in a hydraulic brake banjo housing out of a fire truck. RT series Fullers did OK on vocational trucks, but they have such a big gap between 9th and 10th gear that you have to wait for a down hill stretch to get into 10th gear. Detroits almost need a 13 speed to be drivable on the highway with a single speed rear.
The little dump truck in this picture was had a Detroit with a 5 and 2 and would up in a similar project to what you are doing.
https://plus.google.com/10819770718...5975870034&oid=108197707187186692269
