[tooski]

I just did this on both sides of my 75. I ran the nuts to the end of the bolts and tapped the the nuts with a 3 lb sledge turned side ways. Not much of a swing, but enough to knock out the bolts relatively easily.

[Sirrotica]

Or, you can remove the motor mount bolts, raise the engine up and have plenty of room to remove the upper A arms. More than one way to get a job done.

Knocking the bolts out of the serrations may get you into a situation that the bolts will only stay in place by tack welding them back into the frame mounts. Having run a front end alignment rack you really don't want the bolts spinning in the mounts if you're trying to tighten them back down.