You might notice with Twin Cam master cylinder mounting that it is a tight stretch trying to get to the lower nuts or bolt heads inside the pedal box. In stock configuration the tool to use is a crescent shape wrench for the back side, and a normal wrench in front. The crescent shape wrench is a rare bird if you don't make your own, but there are other solutions.

For myself I would tack weld the hex nuts to the back side of the pedal box to have captive nuts. Another solution is pictured below, making a nut plate to replace the hex nuts (but that gets a bit involved).

An easier fix is to tack weld a crescent shape bar across the bolt heads, install that from the back side, and put the lockwashers and hex nuts in front.