Once you get the guts out of the body I don't think it will be very stiff. And if you welding a cage why not do frame connectors?
I don't follow on the J-bar(?)..not do I understand what they don't want you to brace..you'll have to have something to hold the body skins.
As far as shifting, no, we ran 1/3 mile asphalt and 1/2 mile dirt. We did not shift for either, ran a three speed Saginaw in 3rd. I don't remember our asphalt gear but we ran 5:67 on the dirt. We would turn 58-6200 rpm.