18 ga. seems to be plenty heavy, and even 20 ga. if it's properly supported, or bead rolled to add strength. When I built my Austin gasser I used 20 ga. for everything, except the firewall. I went overboard on the firewall and made it from 16 ga., as I wanted it really ridged. But in hindsight I wish I'd done it in 18 ga. too, as with the offset in it for distributor clearance it would have been plenty stiff.
I used 20 ga. for all the firewall and tunnel on my Falcon gasser, and built a 1" box tubing frame to attach it to. It's extremely strong, and no flexing at all. The 1" tube makes a great support framework, and 20 ga. is plenty with the 1" tubing.