VHF & Transponder antenna

I bought a Cassutt (Primarily tube, fiberglass and fabric). It has a
light-weight BNC battery, starter and alternator, and a old Grand
Rapids engine monitoring system but no radios. I wish to install a
panel mounted MicroAir Radio and Transponder. I think I can figure out
the wiring of the radios but the only aluminum on the airplane is a
12"X11" aluminum belly pan.

Can I use this as a ground plane for both the VHF and transponder
antennas? If so, where should I position them with respect to each
other and the edge? With regard to the antennas, a transponder antenna
came with the MicroAire Transponder, and I plan to purchase a VHF
antenna from aircraft spruce. Aircraft spruce however, has many
different designs, brands and of course prices. Which one would give
me the best performance for my application? I have about 21" from the
belly pan to the ground in the three point position.



Any help you could give will be greatly appreciated