I have 3 different antennas for my MXT275 GMRS radio and 2 of them simply mount to the mag mount I have mounted on the roof centered just in front of the roof rack and my Maxtrax. The cable i ran through the door to the roof.
I use the Ghost antenna up there most of the time so I can fit in garages as well as my own garage. But i also keep the 6db whip antenna handy so i can swap out for my adventures where I need longer range. Easy to unscrew and swap.
IMO its important to have both since there are tradeoffs of gain vs radiation pattern when off-roading. Can't hurt to have the 3 dBi option for coms when you're on uneven terrain and need to reach the rigs around you, then the higher gain options for once you get to flatter and higher ground and can take advantage of the range.
I have my MXT275 mounted here.
This is the cheap one my MXT275 came with that I mounted initially and left just in case.