Chassis Stiffening

I think you're on the right path. I would do the inner fender braces AND the 73-76 fender to firewall braces.

The big advantage to the US Car Tool kit, aside from being precut and ready to go, is that it is pretty discreet. A coat of rattle can undercoating and most people wouldn't know it isn't factory.

As others have said, decide which class you want to race in first before going too nuts on chassis mods. Have you been to any of your local events? I would suggest checking out the local scene and asking questions. There may be some "old iron" guys with some advice, even if they drive Mustangs.