I guess it depends on what you mean by 'fit'?
The frame and brackets, pedals and mechanism are all very similar so a properly designed 1/2-ton kit should bolt into a 3/4- or 1-ton just time. Depending on the other modifications you might be making sometimes the 1/2-ton kits won't fit right in a 1/2-ton.
I recognize your name but don't remember the details of your truck and project so can't comment on what problems you might need to expect.
The second set of issues is that the 1/2-ton master cylinder is not as large a bore as the 1-ton. You really want to maintain that bore size to get proper braking performance. Since you are changing the brakes on the rear that makes the problem more complicated. You want the right bore size to provide enough fuild displacement to activate the rear brakes, and you want it to happen in sync with the front brakes.
I forget what you are doing on the front.
If you search for DOT 5 you'll find a number of threads debating DOT 3, DOT 4 and DOT 5. Personally I've ended up on the DOT 3/4 end every time I've looked at the issue, but others are convinced DOT 5 is the right answer.
If you keep your system sealed and replace the brake fluid as recommended by the manufacturer I can't imagine DOT 5 having any advantages over DOT 4 in a street vehicle. It does have disadvantages.
