Here is a brand-new product from Reidspeed. This comes with two in the pack for V8 applications. This is the perfect solution for your leads to go through the firewall or custom fabricated sheetmetal engine bay.
This Reidspeed plug will fit into a 35mm hole (use hole-saw) with the alloy plate holding everything in place.
Lead holes are 8mm to accomodate various sizes and brands of leads. The plug itself is made from plastic to allow you to drill out the holes to suit your leads.