It appears disc brakes for front axles are getting hard to find, at least in my part of the country.
I put P3500HD front and rear disc brake axles under my ‘37 GMC T-18 HD. In looking for disc brakes for my ‘55 GMC 373, I used a D-80 I had laying around for the rear. I found a Snap-On truck that was built on GMC P 3500 HD running gear, which, unfortunately I was unable to obtain.
However ‘91 to ‘01 Snap-On trucks like this would be another source like in the picture for adding disc brakes to an older GM front axle, tie rod behind the axle. I have another solution for 3500 HD (non P) front axle, tie rod in front of axle, when I have finished the job. Instead of making a steel bushing to adapt original king pin bushings to slightly larger P 3500 HD gear, I found a place in Detroit that makes bushings, for about $12.00 each I got new ones made to fit. GM changed grease fitting locations from side to end of king pins, so need to allow for proper lube, whichever locations one has.
I have never found a P-30 front axle, so have no info on that conversion.
'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires. '47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle. '54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed. '55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.