The V8 swap is pretty simple- - -find a 55-62 V8 truck bellhousing and it should bolt right onto your existing center crossmember. The SM420 is a direct bolt-up to that bellhousing. The frame should already have the appropriate mounting points for a small block V8 since it was an available option for your truck. Some of the V8 trucks used a horseshoe-shaped front mount bracket that bolted onto the front of the block below the water pump- - -all small blocks have 4 bolt holes for that mount. It uses a single large rectangular rubber mount pad that fits on a flat spot in the front crossmember.
An engine that roughly corresponds to the 235/261 situation is the small block 400. It has the same external dimensions as a 350, with a longer stroke, bigger bore, and 50 more cubic inches. The cylinder castings are siamesed like a 261, and the heads have the same type of "steam holes" drilled between the combustion chambers. If you opt for one of those engines, be sure and install a BIG radiator- - - -they can be prone to overheating on a hard pull. There were a very few small block 400's made with 4-bolt main bearing caps. Good luck!
Jerry
