As long as you have one, I'd go with it, I can't see a big difference between them myself. How are the cab bushings as well as the cab mounts, the fronts usually rot out, allowing the cab to sink, putting a load in the steering shaft, would wear the rag joint and bearings out quick?

 

Look on some trucks in a scrapyard or wherever. The closest year to it with power brakes, to see how they were set up. I know there will be a brake safety switch, I think the proportioning valve is seperate from that but I'm not sure.

The manual I just checked noted that on early trucks except F350 there is a differential pressure valve with a pin that must be pulled out when bleeding the front brakes. On early F350 the pin has to be pushed in.

The cheap and sneaky way to find out if the valves are different is to call a parts store or go online looking at parts, run the valve by them as a non-power brake truck first, and then a power brake setup. SEE IF THE PART NUMBER IS THE SAME!