I have thought about this for a while.

I just stumbled across this.
http://www.watsons-streetworks.com/billet_firewall_trim.html

But in the past I wanted to use a combination of these.
http://www.route66supply.com/store/page19.html

1 of the 100 amp connectors for power into the car and a 12 circuit or so for power feeds in and out.