Tim
I would run a single wire two steps heavier than the one supplying the ignition switch. the other end would be a busbar or as wrenchbender suggested( a better ideal) a aux fuse block. Goal 1 - switched power, Goal 2 - get it away from wire cluster of ignition switch, mount anywhere you can access it and be out of sight. Also put a fuse or fuseable link inline between ignition sw & aux box.

Suggestion 2
Put a relay to ign sw and run power straight from battery to aux panel. Wire like a cooling fan relay. 30a is pretty standard , I have seen special applications up to 75a. Still meets both goals. This link shows how a relay works and is wired. I suggest the 4 wire EASY to wire relay and pigtail usually about $10 total

http://www.bcae1.com/relays.htm

Good luck
Brad

http://s320.photobucket.com/albums/nn343/brsdb3381/Tuesday_MY_56_Chevy/

56 3100 6 cyl