The correct grade of steel for classic car patch panel fabrication depends on the application and the welding process you're using.

For exterior panels — floor pans, door skins, quarter panels — 20-gauge mild steel (approximately 0.036 inch) is correct for most American cars of this era. The factory used 18-20 gauge for exterior panels and 18 gauge for structural panels. Using heavier material than the original is not an improvement — it's harder to form, harder to weld, and can create stress concentrations at the weld line.

For inner structure — inner rockers, frame rail patches — 18-gauge is appropriate. For reinforcement gussets — shock tower repair, torque box patches — 16-gauge provides the required strength.

High-strength steel (HSLA) should not be substituted for mild steel in patch panels without understanding the welding implications. It's not more durable for this application; it's harder to weld correctly.