I'm running an A15 at the moment. The 60 series is plenty strong enough for the level of tune.

I would have to ask if they replaced anything other than bearings in the 63 series, my only reason for using it is as mentioned that it has closer ratios.

Up until about 5 years ago I used an A12 coupled to a 56 series box. As that motor was belting out 80 rear wheel horsepower the small box wasn't an issue. While the A12 does have a slightly better selection of parts it's not enough to offset the gain you get with the larger displacement. With that said I have a fresh A12 short block that I'm holding onto as a back up.

One of the Japanese companies (Kamerei , Cusco, can't remember) said they were considering making a close ratio set for 60 series 5 speeds but haven't heard anything back yet.