Options to stiffen chassis 69 Firebird?

[Nativefx]

I've been thinking about my project I've been working on and what I'd like to use it for. It will be mainly a street car, but I am hoping to take it out on some big road tracks a few times a year.

I've already decided and purchased the Ridetech Air Ride setup and a Thompson Motorsports built forged LS3 (roughly 550hp) with T-56 magnum which have already been installed in my stock subframe. Using a Baer full floater rear end with a 9" Moser housing and Strange 3rd member True-track and 3.89 gears. We are currently working on the mockup before finishing the replacement of quarter panels, roof, and tail panel which are just tacked in place. Everything was sand blasted and smaller repairs done, while also smoothing entire fire wall. This is just some back ground of what we're doing.

My question is, what are some of my options to help stiffen the chassis?
Because I finally decided Air Ride was a must and not knowing of any aftermarket options that was designed to use Shockwaves at the time, I decided to use my stock subframe and already had it modified, sandblasted & powdercoated. I already have Hotchkis subframe connectors, which once finished will be welding together rather than just bolted. I'm not sure at this point that I want to install a full cage as the rear seating is already limited, and I'd like to be able to get the kids in the back for short periods of time. I've been learning as I go along, and this is something I hope I can get some advise from more experienced builders or people what came into this same dilemma as they were having their cars built? I think even on the street, with this drivetrain, a stiffer chassis would make for a much nicer setup. Thanks for any help!

PS - I was hoping to post some pictures of where my car is currently in the build, but Photobucket now wants $400/year to 'host' images on the site. Any other options I can use?