You shouldn't have to worry about the ground plane. The only thing that will realistically be affected is the RF "coverage" pattern. You are correct, the middle of the vehicle is the most ideal spot for coverage pattern, but does not really affect "ground plane". Think of your vehicle chassis as a mirror. Your antenna RF "bounces", more or less, the signal off of that mirror.
Something like this is what I'm getting at...
View attachment 32546
Anyways, you shouldn't have any issue. BUT... just make SURE that you have a good contact to your vehicle chassis "ground", if you will. Some people, not talking about you, but some people make the mistake of connecting their antennas to a "metal" part of the vehicle, but ultimately that metal piece is mounted to some plastic piece and THEN to the "vehicle chassis". Thus, that metal piece is now insulated from the vehicle ground itself, if that makes any sense?
Finally, I don't care how many people tell me that "stubby antennas work"... BS!!! Don't get me wrong, they work, just NOT as effective as your more typical antenna. It's the TX that is going to be affected the most.
Anyways, I hope this helps a little bit?
Oh, and btw, it is always still good to check your SWR with a meter in either case to make sure that you're within reason. Don't assume that everything is "peachy" after setup. You don't need to get a super expensive meter as they all work, more or less, the same when it comes to just checking the SWR. Just make sure to get a meter that is within the frequency range you are and/or going to operate in. The meters have a frequency range, just to let you know.