the Buick 300/340/350 would all be good options but they all have taller decks so he might run into some interference issues clearing the exh manifolds, steering shaft, etc. you are correct that they would open up BOP pattern transmissions from the Dynaflow to a 200r4.
an 80s Buick v6 turbo would also be a near bolt in. there would of course be some issues with routing the turbo ducting. the v6 also has the 300 deck height so it will be somewhat wider and taller than the 215/Rover.