I would say that your plan to put the VHF antenna on the side of the frame is a good one, you should get a reasonable distance from the RTE. Having them both on the radar mount might be a bit too close I suspect.
Could you bend and drill a piece of stainless sheet that you can weld or otherwise fix to the frame, to accept a mount something like number three on this page:
http://www.channelribs.com/gear/pacific.html