Repairs while on Jack Stands?
 

'64 Lemans convert, so a lot of flex on lifts. Both front fender lower rear cage nuts have spun. I have to cut out a small at the bottom of the cowl, fix cages, and weld back in to repair. Car is currently on jack stands front and rear to frame, wheels are hanging. I am thinking that it will be fine to do the repair, and would like the extra space to do so. The cowl is attached at so many points, can't see that anything would shift in this repair. Any thoughts? Thanks in advance.

I would move the jack stands to the lower control arms in the front and the rear axle.

To repair what you described, don't be concerned about where you place jack stands. I've cut and welded cars together for decades and made the same repair you're doing. Zero risk of something shifting. if you were cutting a rocker in half, yeah, watch out.