At what HP level does frame/body damage become possible for a 64' malibu convertible?

The only thing I can think of as an issue is the rear crossmember cracking where the upper control arms attach.
It was occuring on 4sp cars for the most part. G.M started to put braces in to stiffen it up. You can get these from Ground-Up maybe. I know other restoration suppliers sell these.
Sorry it didn't answer the question as to max hp.
It's a common weak part of the A-body frame.
Especially when torque is involved.