Is there a rule of thumb for distancing your receiver battery from ignition components. The plans for my Cap say to put the receiver battery at the top of the engine box. This will place the battery pack very close to the ignition battery and module. What is best? Everything else is seperated by at least 12 inches Thanks in advance

12" is the rule of thumb. Typically you want the 2 systems as far from each other as possible. The best thing to do is put everything together and see exactly where you need your rx pack. Then make decisions based on that. If your rx pack needs to go where mentioned then do extensive radio checks with the motor off and on! Also little things like a ferrous ring on the rx pack wire helps.

As DD said the general rule is 12 inches.
If you have a good ignition and good installation, there is nothing wrong with having them close. Many installations have the throttle servo on the engine box and there are no problems. Just make sure that you have solid connections and good maintenance and you will be fine.