Rammer - Welcome to Stovebolt!
I'm guessing the info you mentioned reading about this swap came from this
Legacy Driveline Tech Tip? To answer your question about where to begin looking for the sought after 4.10 center section, go back to the Tech Tip and scroll down to the section on "Finding the correct Donor". It details which trucks you want to be looking for as a donor source. When you do locate one of the potential donor trucks, look for the Service Parts Identification Decal (SPID) that should be on the inner face of the glove box door in the cab (see pic below for a sample). You are looking for a Rear Axle with code "HB8" as that designated the truck was equipped with the optional 4.10 rear axle. The same "HB8" code was used for 1968-72 C/K20 & 30 Chevy trucks for this option.
As an alternative to seeking a donor truck, you can scour your local Craigslist and FB Marketplace ads to see what may be offered. Or you could post an ad of your own in our
Truck Parts Wanted forum and see if anyone has what you seek or can provide pointers to an offered item.
