Brakes

Not a good idea. You need rear disks with the internal parking brake set up.

The parking brake on my f250 hasn't worked in 8 yrs, I just need the better braking and can't afford $800-$2000 to get the conversion kit

Find a F250 rear axle with the disk brake set up at a salvage yard.

Since almost all the braking (unloaded) is done by the front brakes you will not see much improvement in unloaded braking.

F_O_R is right the front brakes do roughly 75 to 80 percent of your braking I really don't see any advantage in the swap

I agree poor idea and it will likely throw the vehicle's brake bias out of whack whick could cause an accident. If you are pulling something heavy enough to over power your brakes then the thing you are pulling needs it's own brakes.