The 223 six of the mid to late fifty's were not very good engines, had several of them in work trucks. Properly taken care of the 292 & 312 V8's were excellent engines.
If the owner wants to keep the truck some what stock looking I would suggest the 300 CID engine from the 1970's, lots of power and longevity.
Add a "Clifford 6=8 " four barrel manifold, w/390 CMF carb, headers, a little head work, a good cam and ignition, will result in a truck that will run of and hide from the average small block out fit. A C-4 trans would make the truck very user friendly.
I had an inline six in a half ton pickup, it had Clifford 6=8 equipment, the thing was a rocket ship, the engine put out over 300 hp on a dyno..
The '59-60 F100's generally have Dana 44's or Ford 9" rear ends w/ 372 ratio.
I currently have three '59 F100 4x4's.