You’ll burn out your inverter very quickly if you attempt to use it as an output control. You have to size the load to control output.
One thing to consider is that while the alternator may only put out 750w, current will also be drawn from the battery.
Personally, I don't do inverters in vehicles anymore after some bad experiences.
A cheap genset can recharge a battery bank pretty quickly. Couple the battery bank with an inverter/charger, and you don't have those high vehicle repair bills.
/johnny
Of course - that’s why you only run a couple of lights and your fridge OR freezer for a while, idling your car for a couple of hours to recharge the battery or while running the fridge.
Steve Harris has a whole interview on the topic on his site.