I'd go for a 261, wish I had know about them when I was in high school. Instead I rebuilt a 235 for my 1 ton (while in high school) at the time I wanted lots more HP, but looking back the 235 was a great decision and I learned a lot.

Doing something reasonably stock with easy to get parts and most of the answers in the service manual is a good idea for a first project. Save the unusual or far from stock engine swap for your second or third project.

For a transmission either the 3 or 4 speed and faster rear axle gears. Neither have Over Drive, and the advantage to the 4 speed is a nice slow first gear and a floor shifter.
Or I'd seriously think about a T5 5 speed OD possibly with stock gears. You'll need to go to an open driveline though.

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-