Perhaps I shouldn't have mentioned "extension" at all because it's not. It's only confused with or misunderstood as an extension and I may have added to that confusion.

These trucks had 2 different front hubs, one type that fits dual wheels, and another that fits single wheels. That's it, no extensions.

Ask the folks that make the disc brake setup what will work.

Looking in the parts catalog the backing plates are the same for single and dual wheels, as I expected.
I thought drums were also same, and they are for 53-54, but for 47-52 they list different front drums for single and dual wheels. I'm not sure the difference or if enough difference to matter.
http://chevy.oldcarmanualproject.com/parts/1929_54/369.htm

I believe the disc brake kit has you slide the rotors on over the studs, before attaching the wheel. this method won't work with the dual wheel hub, a rotor would have to bolt on the back of the hub just the same as the original drum did. Ask the seller before buying.

•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-