#2, no blind nuts. Use fender washers and lock nuts with Locktite. Nuts aren't required to be inside the firewall, they can also be at the front of an engine. Personal choice, application requirements, and usable space determine what goes where.

The fender washers have no barbs to bend over. The fender washers spread force loading over a wider area of the firewall. 1 long bolt has fewer fasteners on the other end to worry about. Easier to set in place and align.