they're buried to far down in there for a measure.. but if you look at that pic and compare them to bolt head, they aren't that big. appear to be between 5/8" and 3/4". its not that critical.. you just need to raise it some to help that angle and distance... the cable adjustment will make up for any little variance in that height...
i'm telling ya, flat washers will be the simplest... take way less time than cutting down the right size pipe.. plus it'll give a more stable base for the bracket to sit on...
i'm not gonna tell ya to drive it... i can't advocate that... yeah, sure, if your real close, baby it, and the cable is not too outta whack, ya might be ok.. but i can't advocate it, you can hurt the tranny in a hurry with that cable not right...