I strongly recommend a 235, or a 261 if you can find one. These are both bolt-in swaps. I currently have a 1948 235 truck engine (babbit/ splash-oiler) in my '53 3600. It is a very low-mileage engine I picked up from a guy who was swapping a small block into the 1.5 ton truck it came out of. I could not be happier with the way my truck runs, and the power this older 235 has! I use my truck as an actual work truck, and sometimes do drive on the interstate, as much as 70 miles at a stretch. I'm running a Carter YF carb, btw, and I also have the coveted 4.10 rear end gears. This truck will easily do 65-70 mph (with the tallest tires I could fit, I believe they're 235/75 R 16s).
230/250/292 will fit, but if I remember right from reading posts here, it will require moving the radiator forward. In my opinion, this is a better option than a V8. At one point, years ago, a mechanic friend talked me into putting a 350 sbc in this truck, after I had very naively thrown a rod in the late 50's 235 it had when I bought it. Sbc swaps involve moving the steering gear and fabricating motor mounts. In my case, I never got the 350 to cool properly and wound up fatally overheating it and ruining one of the heads. If I knew then (roughly 20 years ago) what I know now, I would have just had that first 235 rebuilt. In any case, after my ill-fated few years with the V8, I went back to Stovebolt 6s and will never look back.
