I'd be scared to try to remove a ridge with a rotary stone/bit/abrasive wheel, whatever. To maintain the tolerance you need for eight bores? Without botching one up? I'd go back to the reamer.

The mistake I see people make with a ridge reamer is they turn it with a ratchet or a breaker bar. This imposes a side load on the tool and just makes it harder to maintain alignment with the bore. I get above the block (use a small step stool) and use a T-handle for the reamer so everything stays aligned. Just my