The spindle boss holes are almost certainly:
1. Threaded for indexing (probably a UNC thread)
2. Unthreaded for locking the spindle.
Find a steel bar that's a snug fit in the hole and rotate the spindle until it locks. If it doesn't lock, it may be that you need a slight reduction in diameter on the end of the bar. Once firmly in place, get someone to hold it there, hold a close fitting spanner on the hex and give it a sharp whack.
Don't even think about using screwdrivers and Stilsons!
1. Threaded for indexing (probably a UNC thread)
2. Unthreaded for locking the spindle.
Find a steel bar that's a snug fit in the hole and rotate the spindle until it locks. If it doesn't lock, it may be that you need a slight reduction in diameter on the end of the bar. Once firmly in place, get someone to hold it there, hold a close fitting spanner on the hex and give it a sharp whack.
Don't even think about using screwdrivers and Stilsons!