Just for some testing. Measure your existing chain rings and see if you have one at the 50 mm chain line spacing. If you do shift to this ring and then shift your back gears and see if you have any rubbing on the front derailer. This should give you some good information about where the new chain line needs to be located.

Replacing the BB may be tough unless you have a vice or the right tools to take out the BB and put the new one in. I think that these BB's take a good bit of torque.