You need to look at a lot more than just "bearing size." You need the ID, OD, thickness, type, and probably what will be the most important thing, the length of the center stem. I hate to be a Debbie Downer, but I bet you a quarter the triple tree you want to "bolt on" has a much longer center stem than the original XS stem.
Does that mean it can't be done? Of course not. I have an '81 XS400 in my shop right now undergoing a ground up custom build. It is getting a massive GSXr front end swap.
Make no mistake, the swap is not a bolt on! It's requiring a LOT of custom machine work. Machine work for the triple tree, for the brakes rotors (what is pictured isn't being used... going with larger drilled rotors), for custom stops, for the speedo and tach mount... like I said, this is a ground up custom professional build.
NOTE: For whatever reason I cannot upload a pic. Check out pictures of the bike on our Facebook page @ Evil Monkey Kustoms