It looks like the last bolt is impossible to get at. Is it accessible by taking off the hood?Parts from the dealer are outrageous on these starters. I don't even bother to fix them anymore. The last few I replaced were in the $50 range from the local tractor rebuilder. They are new aftermarket but have worked just as well as the OEM unit they replaced. Just an option to consider.
Taking the hood off will not help. Use a 3/8" drive, 9/16 swivel socket with 8 to 10" extension and ratchet. Before loosening this bolt, put one of the other bolts back in. That will make it easier to remove the difficult bolt.It looks like the last bolt is impossible to get at. Is it accessible by taking off the hood?