Everyone on here is providing excellent advice. I'll just add an endorsement of the 235 (or 261 if you can find it) route. When I was young and stupid, a mechanic friend talked me into dropping a 350 into my '53 truck. Worst mistake of my life! They don't fit right, you need to fab or install motor mounts, move the steering box, cut the firewall, etc. etc. Major PITA. If I had known then what I know now, I would have gone straight to a 54-up 235 (or 261). They bolt right in and provide a nice uptick in power. With a rear end gear change (and/or bigger tires), you can get acceptable highway speed. 235s are plentiful and not terribly costly to rebuild. I have 3 235 blocks and 1 261 sitting in my shop now--my inventory for future projects. All were acquired locally via Craigslist or Facebook Marketplace for $100 to $300 each. In my 46 Chevy car I have a fully rebuilt 235 (bored out .60, which, if my calculations are correct makes it a 248); I spent about $1800 getting it rebuilt at a local shop--that was about 3 years ago, I know prices are up some but still not crazy for essentially a new motor. In my 53 truck I have a '48 big truck 235 that I picked up for $250 and just dropped in because it only had 5,000 miles on it! Best purchase I've ever made--my truck starts easily, has plenty of power, and will do 70mph with my 4.10 rear and big tires.
Conclusion: stay with a stovebolt 6! They are great motors, they are what these trucks were designed for, they are still plentiful, and they are relatively easy and inexpensive to work with.
Best of luck--let us know what you decide.
