Tip #1: Be *really careful* with this. Make sure that you always have a cutoff for both the drive and the engine... the potential for danger here is huge. Make sure that if any component of the control system fails, then the engine cutoff is triggered (that includes whatever you use to trigger the engine cutoff)
Although the voltages match up between the batteries and the alternator, and both likely have the necessary current sourcing capability, the alternator produces AC which won't work. Sorry, but you're stuck with batteries (it isn't reasonable to use an inverter to power the wheels, either).
Whether or not you could get by using a lower-capacity battery depends mostly on how long you wish to use it for at a time. FRC have batteries in a similar capacity range, and I find we can often run for 20 minutes or so at a time off of one battery (intermittantly, granted -- not at competition). If you set it up and it takes forever to drain the batteries, then you can use smaller batteries
