I'm late to the game but figured I'd chime in, as I did 25 years ago most of what it looks like you're in the middle of. I put a 350 in my '53 3600, keeping everything else stock (including 4 speed SM420 trans). As I'm sure you've learned, the steering box will need to move about 2" out to clear any kind of conventional exhaust manifolds on a SBC. Also you'll need to cut out part of the firewall to clear the distributor (and also the rear of the rocker arm cover, IIRC). My 350 gave me plenty of power and was fund to drive, but I never was able to get it to cool properly, and so I overheated it regularly and ended up with a pretty massive engine failure (one of the valves snapped off and was actually embedded in the piston).
At that point I decided to go back to the stovebolt 6 that the truck was designed for. I ran a bone stock 216 for several years with zero problems. I found a slightly used 1.5 ton truck 235 (splash oiler) for sale near me for $250. The guy said it only had 5,000 miles on it. I put that engine in and have never looked back! A well-running 235 has plenty of power for this truck, and with 235/75 R16 tires and a 4.11 rear end transplant, I can cruise comfortably between 65-70 on the highway.
