To remove the master cylinder assembly from the frame, all you'll need to do really is get those mushroom bolt heads off (unless the frame itself is threaded, which I doubt). You're halfway there since they're mangled already. 
Getting the remainder of the threaded bolts out of the master cylinder assembly afterwards might then be done a little easier on a work table. Dose them liberally with PJ Blaster and leave them overnight, then warm them up a bit with a heat gun. There may be enough of a stub left on the bolts after the heads are off to get a purchase on them with a vise wrench.
Getting the remainder of the threaded bolts out of the master cylinder assembly afterwards might then be done a little easier on a work table. Dose them liberally with PJ Blaster and leave them overnight, then warm them up a bit with a heat gun. There may be enough of a stub left on the bolts after the heads are off to get a purchase on them with a vise wrench.
