Hi Sid,
I've seen people use different areas on the rear panels for antennas. I have a 1960 Convertible and mine is set 3/4" away from the trunk/quarter-panel divide, and 3" back from trunk/body divide. The exact distance is not as important as each antenna being uniform on either side. Also, just one antenna needs to be wired. The other is referred to as a "dummy" antenna and is basically just for looks.
I have a problem with my outside mirrors, in that I can't see out the passenger side unless I slide way over. I've seen some of these mounted parallel to the vent windows to avoid this problem. I've also seen them mounted on the back end of front fenders... which I didn't care for.
Good luck with the resto job.
Rich