i would not be surprised to find different castings OEM from the factory, there are many years with multiple head castings used that don't have any significant difference between them.
If so, Is this a problem?
could be. if a head required replacing, it was likely due to a burnt exhaust valve or significant overheating warping the head so far they couldn't mill it flat.
inspect the rest of the engine for signs of over heating like discoloration or an apparent previous rebuild.
i believe that any Small Block Buick distributor will drop into the block, all the way up to the 350ci which gives you an HEI option. although the HEI cap is really wide compared to the points cap and might have interference issues.
you can also try something like the BritishV8 forums for 2nd hand parts as the Rover 3.5L v8 engine is just a metric version of the 215ci. let them know WHY you want the parts and they should have no problem steering you in the right direction.