It can vary from installation to installation, but your arrangement sounds okay to me, as I've never been able to have that much space between my onboard radio stuff and my G45, and so far I have never had a problem.

The smart thing to do is to position your equipment in the location that suits you best and do a range check with the antenna collapsed and with the engine off and then on.

Anything over 150' with the engine off and no more than a 10-15 % reduction in range with the engine running is good to go.

Karol