If there are any bolts that come thru firewall threads, you can use longer bolts there, which create a stud inside. With spacers and nuts, you can mount the box to the "studs".
OR...You can find structure/holes inside that you can attach a bracket to. Attach box to your fabbed bracket.
OR... You can create your own studs with carriage bolts. Head to outside of firewall for a clean, flush look. This bolt is sometimes called a "Stovebolt". Apropos aye?
OR... You can rivet a bracket inside of firewall, rivet head on outside of firewall. Mount box to the bracket.
Your limiting yourself with the idea of no holes in firewall. The holes that you drill for the box will be filled by a bolt anyway. Is this a 5 star restoration? If it is, the box would be mounted outside on firewall.
I would assume you realize that mounting to firewall is very practical. Most boxes in the Stovebolt world are mounted to the firewall. Many kits include a bracket and instructions to do so.
Your harness runs to engine/front lights and to dash (gauges, switches, column are on the left) and to rear lights (only 4-5 wires). The central protected accessible distribution location is....... inside, upper left of your left foot.
Tim asked you what year, assume your username says it.
Chevy finally came to their senses and mounted the acid filled, fume generating battery outside cab and some corrosion sensitive electrical parts inside cab. Took them awhile. Electrical, and especially automotive electrical, was an emerging science.
