The source is NAPA, that's their part number.
I just called and ordered a kit myself, 70 some dollars and no shipping if willing to wait a few weeks for the normal NAPA trucks to bring it. As of today, 6-25-19 They have one in Atlanta, Memphis, Oklahoma, LA, and Utah among other places I assume, and 5 kits at the manufacturer's distribution warehouse.
It is still a good part number and when those are gone more will be available. My parts guy at NAPA recalled that part number was (in past years) a very common kit, even before looking it up in the computer.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-