The "rule of thumb" is to try and keep all Ignition related wiring, switches, etc. at least 8 inches away from anyhting related to the radio.

As for the throttle servo. Some guys have no problem mounting them directly on the firewall, or to the engine box. I do this too and have little or no problm. On some of my planes I've used fiber optic isolators to seperate the throttle and ignition battery......
What kind of engine do you have? Magneto or Electronic?