You'll need a new antenna, a CB antenna wont' be resonant on the 460mhz frequencies. As for reusing the old mount I would just replace it with an NMO mount as the majority of antennas you will use for GMRS will be NMO antennas. For GMRS use I suggest going with a business band antenna in the 450-470 range since that's where GMRS is. A 70cm ham antenna will work but is less than ideal.
Here are my suggestions for an antenna. I've attached to a graphic that shows the radiation pattern of different types of antennas and you can get an idea of what would work better for you in different terrain. Don't get too wrapped up in antenna length or dB gain. Higher gain or a longer antenna isn't always better in many instances. Even though a longer antenna might look cooler it may not be the best tool for the job at hand.