The head of a RS500 is exactly the same casting as any late 2wd head with a 3 bolt thermostat mount on the front (I have one in my shed for spares). The only difference is two holes in the mating face where the plenum attaches for two small dowels to sit, and the fact that the inlet tracts were machined out very slightly at the entry point.

The pistons, rods, crank etc are all standard 2wd items only the oil pump is different which was the prototype version of the 4x4 pump. Same goes with the thermostat housing this was an early development of the 4x4 one

The only special part of an RS500 engine is the block and all its bolt ons - technically the 4x4 internals are superior to the RS500 as they have a better piston design reducing piston slap and oil spray arrangement.

All fact not hearsay