You're gonna get a lot of opinions on this one, but my favorite is under the cowling. Make a shelf aft of the engine baffle on the centerline and run your coaxes together through a fireproof firewall flange back into the rear of the instrument panel. That will keep your glare shield clear of 'antenna clutter' and as far as I can tell there is no degradation whatever from the thin fiberglass of your upper cowling on signal strength. Just don't have heat resistant foil over the part where the antennae are and you'll be just fine.
Lee...
