Are you planning on keeping the 3/4 ton axles and 8 lug wheels, or going to something more modern? The most common rear axle ratio in a 3600 is a 4.56:1 ratio, which will have any engine screaming at highway speed- - - -six or V8. A 700R4 or 4L60 automatic trans with overdrive will bolt onto the back of the 250 engine and help out on the cruise RPM, but the throttle pressure linkage on the 700R4 will be problematic, and the 4L60 will need an electronic shift controller ($$$$$). You'll have to fabricate everything- - - -engine and transmission mounts, fuel lines, radiator, throttle linkage, etc. The old bank account will need some CPR before the project is completed- - - -if it ever gets done.
Jerry
