Frame inside cab - legal?

[D13]

Really odd question. I'm looking to do a channeled, altered wheel base 80's square body. The best way I can see to get what I'm after is to run the frame members inside the cab, along the rockers. The firewall and rear wall would be flanged and welded to to the frame (probably 2x3 or 3x3 tube) and a roll cage added (which is actually what drove me to this idea). I want the truck to have working heat, etc, which is why I'd like to keep the existing firewall, as all those little bits are already mounted up.

When I read the (apparently deliberately) confusing IRHA ruelbook, there is almost nothing in it for ET brackets, specifically for vehicles that don't fit neatly into one of their classes. Which is where something like this would have to run since the engine is under 300 cubes.

Side note - IHRA because the just reopened Onondaga dragway here in Michigan, and since the noise from that is what got me into cars, I'd liek to pay it a visit with something the Michigan Madman (EJ POtter) would ahve appreciated.

Sooo... does this count as a firewall modification? Because if I read it right modified firewalls require a whole bunch more stuff and I'll just run it on the street. Kind of funny because I can run rotted rockers and floors patched with scrap road signs, cover it with carpet and apparently pass tech.