9.5" wheels are plenty wide for a 12.50 tire. Most people run 12.50s on 8" wheels. I run my 33x13.50s on 8's and they're just fine. For off-road use only, you can find plenty of people who run 14.50s on 8's; you can even find a few that run 35x16.00 Boggers on 8's. A lot of tire places won't mount a tire that's more than 4" wider than the rim though - but, your 12.50s on 9.5s will be just fine.