Run the stock one, see what ya think. If the rears are overpowered, consider an adjustable valve.

 

Awesome, thank you!

 

should only need to change the proportioning valve if the new axle has disk brakes. the shocks mounts should not be an issue. i think even the driveshaft should be fine. it is not like you are going to a dana 60. you will likely need get rims that are like on new trucks for the axle which will require spacers to run the matching rims in the front and it will be hard on the spindles. you could always ask for measurements. it would be easy for them to measure from backing plate to backing plate and you can do the same. you can also he measurements from leaf spring perch to leaf spring perch. it will help you figure out how different they are.

 

I don't have discs, but in case I do, I picked up a generic brass disc/disc prop valve from a hotrod supply shop - similar to what summit racing sells. I'm not sure if the stock master is appropriate. Will jump off that bridge when I get to it.

The newer axle should be roughly 70" wide instead of the original 65" in yours. This is measured across the wheel mounting surfaces. Backing plate measurements aren't reliable, as brake designs vary so much.

Edit: spacers with later wheels (positive offset) aren't hard on front spindles or bearings; they cancel each other out and tire ends up where its supposed to be.