Early-A Convertible Chassis Reinforcement

Sub-frame connectors will do the most.
Up front, you could just bolt a couple of struts between the firewall top flange, and the fender bolts, as was done in some 70's cars. I welded up a shock tower brace that was my usual brand of overkill, and maybe heavier than necessary. I also added a roll bar, so my 67 convertible is super-tight. I also bolted a piece of tubing across the radiator shroud, low down.
Interesting cross-section pics, 65Val, thanks.