I would mount the porportioning valve either beside the MC, or in a convienent spot on the frame. You will probably set it once and forget about it.
I used one on the last truck I put together, it was under the cab outside the frame, I may have gotten out and adjusted it 2 or 3 times when I first got the truck on the road, then never touched it again.
I can't think of a good reason to mount it in the dash, as close as I would get to in the cab is have the lines outside and just the stem and knob poke through the floor board. But then you may want to consider the cab moving on the mounts and flexing the line, overtime could cause a crack. now you have another reason to leave it outside the cab on the frame..
For your truck I would suggest 1/4" line back to the rear axle, then 3/16" to each wheel.
Front probably 1/4 to the T, then 3/16 from there, or could do it all with 1/4 if you like.
Really should be fine with what the factory setup was, as you still have original brakes right?
Either power or manual brakes, same line size.
Grigg
