When I did my box replacement I was able to get a Drake bracket and grind and drill out the rivets of the stock unit (doable but not fun) Bolted the new bracket in and it was all done without removing the fender or the motor. The attached link was helpful in the rest of the process. It's worth every bit of grief you have to go through.
https://www.goldengatev8.org/tips/St...ngGearSwap.htm