While putting my upper control arms back I got the nut on one of these brand new bolts seized so tight even my impact could not back it off. Ended up cutting it in half. In the process I stripped the knurls inside the frame. Should I order new ones and just hold both ends to torque, or just replace with grade 8 at this point?

I got it this far, went from typical lock washer resistance to locked in about half a turn.

 

If the bolt can spin, it makes it more difficult for the alignment guy to loosen/tighten the nut to install/remove shims..

 

It may be inconvenient for the alignment guy but it will work.

 

Would it be better to get new knurled bolts and then tack it in place when its all done?

 

You can tack the GR8 in place.

 

Cut it, drift out the stud and replace it. It's really easy to do. The Vette venders sell the correct knurled stud n nut. Fusick, Year One, OPG may sell it too.