I have a X block and they certainly look very similar to a R block. They are missing the extra cylinder head bolt holes that are found on the R blocks. The one I have is a four bolt bottom end. I would think the bottom end would be similar strength wise.
If the price was right on a X block buy it. They are a huge improvement over a factory block. It would be a good block for a boosted application but the R block with the additional head bolts would probably be even better.