My bracket came in from Boyds. The bracket to block bolt wouldn't go into the block. I took the following steps below to clean the threads. It was a slow process, but it worked.
I took a .30 cal bronze bore brush and twisted it in the threads. A haze of rust dust came out. I squirted PB blaster in there and worked it with the brush. I cleaned it with brake fluid. I then sprayed an orange kitchen degreaser in the threads and let it sit for a 10 mins. Then worked it with the bore brush again (I cut the tip of the brush off since the brisles were mashed down). Cleaned it with brake cleaner and repeated the degreaser (this time some of the brissles fell of and lodged in there). I tried the bolt and only got a couple of turns. I took a M8 x 1.25 tap and through a combination of the tap handle, my hands a and a pair of pliers, I got the hole cleaned. I would turn it till it became hard and then clean the hole and tap with brake cleaner. I would spray the tap with WD40 and go again. After three hrs, the bolt fit. I had to drop the starter a little to get the bracket on. I used all Grade 8/meteric equivalent harware. I put a lock washer on the bracket to block bolt and a washer and lock washer on the bracket to starter stud. I took it for a 20 min ride, but didn't have the time to check the install. I will check tonight and go pick up new starter bolts since these have been on without the bracket