If it were me and having some idea just how much trouble (lots) it'd be to install a 7.3 in an AD truck it better be a whole lot better than simply a "good motor" to make it a worthwhile project.
I'd be looking for the best engine I could find with little or only reasonable difficulty to install.

The 4-53T I have in my truck is a great engine, I like it and understand it, but it is just a little longer than fits comfortably in an AD truck. My next swap will be a 3-53T in another Ad truck, that'll be a lot simpler with one cylinder shorter.

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-