Frame will handle the NA 496 and not give a problem, but potential handling and traction benefits are there if you were to box it. Just takes about 20' of 1/8" x 4" to 6" ?? flat bar steel welded over the opening. It's been a while since I measured one, I don't remember for sure what width flat bar is needed, you'd have to give that a quick check if you wanted to do it. Makes it more solid like the a-body convertible and El Camino frames from the 60's and mid 70's that came boxed from the factory.
At the very least I would plan to replace all the stock rubber body-to-frame mount bushings with polyurethane performance versions....the old rubber is sure to be dry rotted and loose. This will stiffen things up a lot. With good stiff bushings, the body and frame together help stiffen each other….with dead rubber they can't help each other. Poly bushings and boxed would be the top dog way to go to avoid any need for a roll bar on the street to keep it stiff. more of a sleeper performance mindset, and the way I'd do it.
Yeah, stock rear axle and stock control arms in the rear will either break(the rear axle) or flex and cause wheel hop(the stock arms), so I would plan to replace all that. Springs are probably worn out too.
Roll bar or cage won't be necessary to keep it together, but on sticky tires the motor will make enough steam the car will be fast enough that strip rules will likely require one if you plan more than 1 track day.
you can plainly see the open channel section I this picture: