There are some pictures here that can help.
http://www.vansairforce.com/community/showthread.php?t=146326
I'm working on this now, drilling holes in the firewall. Looks like I'll need two, one 3/4" and one 1".
I'm surprised that none of the engine monitors have a module that can stay in front of the firewall, to reduce the number of wires that have to go into the cockpit. My bundle of EGT and CHT probes alone is about 3/4".