Did you know they make a 12point (9/16 head) X5/8 bolt for bolting on the starter? Just look up 12 point cap screws. Handier for the inside bolt because the 12 point will hang onto a 9/16 socket and long extension during install. May have helped on your close clearance bolt.
Things have changed......
Current best approach for the awkward mounting bolt that required some grinding is to use a 5/8 x 1-3/4 button head cap screw and a cut down 3/8 allen wrench. This method requires no grinding as I have found that some of the China knock-off starter casting are too thin to accommodate any significant grinding without going through the casting.