I tried the ball mount on the quarter panel, but tore it off on a rock. The best place I've found for durability is in the cowl piece, on the driver's side. The one I have there now is part of a bottom loaded magnet mount, I had to drill a hole (7/8"?), and mounted it. The magnet mount part of the base was trash, so I jsut unscrewed it and used the rest. On a first gen, you may be able to rig up something that would look like the OE radio antenae.
The best mount for reception is a single whip mounted in the center of the roof (on a '76 +), I've heard a lot of good things about the Wilsons. On a full convertible, you really don't hav a good ground plane. If you leave the top on year round, you might be able to use a boat antenae that is meant to be mounted to fiberglass.
Pete
'83 K5, 350 TBI (ex 6.2), 700R4, NP208, Dana 60/14 bolt, 4.56s, Detroits, 3" lift, 15-39.5x15 TSLs
'97 Dodge 2500 4x4 CC LB Sport, Cummins 5 spd