I am installing a Highway 22 in a new Fiberglass 41 Willys Coupe. Can the fusebox be mounted behind the dash on the mounting sheet metal using magnets?

The Highway Series universal kits are designed to allow the builder / installer the opportunity to place the fuse box in a location of their choice. Mounting the unit under the dash is certainly acceptable. Mounting the box to a metal bracket or plate and then attaching it to a mating plate attached to the firewall via magnets will have no adverse effect on the panels operation.

I would suggest the installation of #500717 Grounding Kit in your particular project to allow for more efficient grounding requirements and locations. This is especially critical with fiberglass bodied projects.