[DonRotolo]

You can strengthen the assembly considerably (enough to drop down to 0.062 material) by adding some gusset plates, specifically, where the inner plates meet with the front and rear plates (4 locations)

Triangles (2 or 3" long) over or under the 'wheel well' area would be OK, or at the central area if necessary. This will prevent the chassis from racking.

The thinner material would be fine if (and only if) you had bumpers there: Use high-quality plywood and use its strength to your advantage.

If you fear insufficient strength at critical points, like axle mounts, add a metal plate (riveted in place).

Overall an very nice design, looks solid and manufacturable.