Usually belly between firewall & main spar
I have purchased an ICOM A210 COMM transceiver and Garmin GTX327 radar transponder. I also bought a CI-122 bent whip COMM antenna and a CI-101 "ball on a stick" transponder antenna. I plan to install both on the belly of my RV6. Where have others successfully installed these or similar antennae? Also any wire run issues to be aware of or recommendations? Jim Sharkey RV6 Tip-up O360-A1A FP Getting there!!!
Do a archive search, lots of ideas. Spacing (ant to ant) of 3 feet or more is ideal, but often violated with no ill affect.
If you think about it the best place is both on the belly, just aft of the firewall, one on the left one on the right. Try and avoid putting it directly down stream & in-line of exhaust or too near exhaust pipes or gear legs. (Don't know what pipes you have.) Coax runs are minimal in this location. You will not get a full three feet antenna to antenna spacing, but I did this on my RV and there where no ill affect.
Another place is just fwd of the main spar carry through, belly, left, center or right. If you have it on the floor, fwd spar, under your knees, pilot side, you can make access to the BNC connector while seated. You can reach down and disconnect the coax in-flight and connect a short coax connector to a back-up hand held, if that appeals to you. This assumes your main radio dies and you have a back-up radio to use. Its one idea might turn you on. The coax runs are a little longer as you go aft. No big deal. Trying to go aft of the main spar has some access issues and not really needed in my opinion.
You may be able to put one antenna center-line just aft of firewall (battery allowing?) and the other center line just fwd of the main spar. (This assumes you don't have a single center line exhaust.) I can't picture the RV-6 floor structure, but the location of the antennas should be come apparent to you with a little thought. You can go any where on the heavy belly skin is my general advice.
You will get great performance from your modern radio and antenna, even if the antenna location is not perfect or ideal. Ideal is MAX distance from other antennas and other structure, gear legs & exhaust pipe. In general BELLY is best for in-flight Com (which is what you should want). Top of fuselage is generally best for ground communication. However a belly antenna will never give you a problem for ground com at smaller typical GA airports. Its not like you are flying out of JFK, ORD, DEN or LGA. Some times big terminals and long distances make belly antennas marginal.