I can't give a specific answer but I will say that the workmanship and materials are generally better historically on public buildings and bridges, less so on houses and barns and some smaller outbuildings are where the apprentice learned and made his and now her mistakes. In other words, the tolerances depend on the type of building.

4"x6" braces are big braces compared to historic New England framing. Without knowing the engineering, it seems you could cut off another 2" and still be O.K., especially with oak. It matters how long they are, too. 3" x 5" is a standard size brace.

Old frames sometimes have significant variation in the size of the timbers. One barn I worked on had a 12' girt sawn 7" tall on one end and 6" tall on the other...oops. Of course, rafters were often intended to be tapered.

I do not know of any written standards for tolerances.

Good luck;
Jim