That ebay auction appears to be a Rockwell rear axle from a Chevy 3500HD. If I am guessing correctly it has 10 lugs on 7.25" circle, like your truck.
He calls it a "10 bolt" but it is not the GM axle most people call a 10 bolt same falily as a 14 bolt, it appears to be a Rockwell.

It is not a great idea to put Disc brakes on the rear without having them on the front too.

If he has the rest of the front axle, I see hubs and calipers in the picture, then that would be an option too (after narrowing).

For the price he is asking both front and rear axles should be included and in great shape..

Grigg

•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-