You won't find an MTB Octalink BB with the short splines required for the 6500 cranks, they stopped making them 15 years ago. At this point, for a 1× road build, you should be cutting your losses and buying an FC-S501 Alfine HT2 crank and BB
Found the problem, spindle seems to have broken as each side is moving independently...
Think I'll try and find a v1 bb for cheap as this is meant to be a parts bin build! That would make sense in the long run though
You won't find an MTB Octalink BB with the short splines required for the 6500 cranks, they stopped making them 15 years ago. At this point, for a 1× road build, you should be cutting your losses and buying an FC-S501 Alfine HT2 crank and BB