GM sells these engine as emissions-approved OEM replacements that also have a GM warranty. It appears that to keep in line with emissions regs they need to sell a slightly different engine for HD than for the standard 1/2 ton. However, in your case either engine should work fine unless you have to provide proof of emissions compatibility for some reason (not likely for your '64).
I used one of the older style SBC 350 crate engines for my '75 truck since it was a direct replacement, and it works fine. However, I think this newer engine design with Vortec heads and a roller cam would have been a better choice as a starting point. The engine does not have the fittings for a mechanical fuel pump, and you need to use a Vortec intake manifold, but other than that they are a very simple swap.