a friend of mine has a 1975 Highboy with a low pinion closed knuckle dana 44 front axle and he wants to know if he can swap in a 1978 f250s dana 44 axle so he can have disc brakes. i know that the 78 axle is high pinion instead of low pinion but will it still work. or should he find a 1976 front axle that has disc brakes and is still a low pinion.

 

The high pinion will fit, but there are a few of things that need to be addressed.
The first is exactly what you mentioned, the high pinion VS the low pinion. The driveshaft will have to be shortened to even fit after the high pinion axle is installed.
Much easier to make one shorter than longer. Those driveshaft stretching tools are getting hard to come by.
Next is the Brakes. Since the brakes are different, a new prop valve, and different lines will be required, but again this is minimal.
Last is the steering. The 75 would have had power assist, and the later model will have power steering. While the steering arms are different, it does not take too much to mount the hydro ram using the old tie rod, but you will have to get creative with how you attach the steering arm to the drag link, but with minor fabrication, it can be done.
Might be a good time to convert to power steering now.

 

you will need master cylinder made for disc brakes.