Booo yaa! this is the rim you want. This style rim is the standard of light and strong. It comes in a few different guises. the most notable being the Borrianai and Akront from Italy and Spain respectively. These are the most sought after and will cost accordingly. I've found the Takasago from Japan to be a very well made rim and prefer it to the DID version. However, I still run the DIDs on one of my bikes and have no complaints. The key here is the high shoulder. Great for rigidity and it looks superb. Downside: cleaning them is a chore.
for the BSA, it really depends on how wide you want the rear tire but if, for example, you get a 19 front and the 18 rear, its a pretty common combo. the larger dia. front will have a lower profile tire and the rear will inherently have a little taller profile. this makes them pretty close to the same o.d. after fitting rubber. It makes the bike handle quite controllably and smoothly. I, however prefer the look of matching Dia. rims -and nineteens would look the dogs bullocks on your bike! If you get a matched set of the Takasago rims from the XS650 they will be pretty thin for a rear tire. but it can be done