Here is the link for the aluminum ones from the gamma Goats
http://adelmans.com/military.htm A 3-53 would not gain you much power over the 6 cylinder gas, so I did not consider it for to long, if you are going to swap engines there should be some sort of benefit right?
If I was going to use a 3-53 I would look for a cast iron one first, actually more common, it's just the military aluminum ones are plentiful and cheep at the moment on the surplus market.
The 4-53T is, as you guessed, 4 cylinders but otherwise identical to the 3-53 three cylinder, just about 6" longer. The "T" after the numbers designates that it has a turbocharger. The 4-53 turbo is the highest HP 4-53 available. (the marine units should be more HP than the truck engines because they can take advantage of the abundant cooling and burn more fuel and still keep the engine cool.) So, the best bang for your buck, or 4-53, is with the turbo. I figured I was in no rush and I would just keep my eyes open for the 4-53T instead of swapping something else and then wishing I had found a 4-53T in the first place.
The Detroit probably won't compare with the new Diesel pickups for performance, but that's not the point. It'll be different, interesting, fun, and loud in a old Chevy.
4-53T engines are most commonly found in Chevy and GMC trucks of about 2 tons in the 70's and very early 80's. Don't be confused if you find one of the trucks in the 80's with a Detroit diesel but it is a V8 and called a "fuel pincher", that's a 4 cycle and not what you are looking for.
I priced a rebuilt one from Detroit, they wanted about $3,000 for a core charge and then about $9,000 or $11,000 before they would give you a rebuilt one, I forget how much exactly, but more than I though it was worth. A basic overhaul kit for the 4 cylinder runs around about $1,000, plus your labor.
