Mounting to Rotisserie

[Rust Buster]

I'm starting the restoration on my 71 CST Blazer and built a rotisserie. I lifted the body off the frame today and plan on mounting to the rotisserie tomorrow. I braced the door openings and took measurements to baseline things. I prefer not to mount it to the rotisserie by the underneath frame rails...I want to be able access and completely finish the entire underbody without having any mounting hardware in the way.

Is there any downside to mounting the rotisserie frame to the firewall where the frame rails end? I'll sandwich the rotisserie frame and another piece of square tube on the inside of the firewall. This will actually give a better balance overall. On the rear I'll do the same to the rear pan.

Thoughts?