i ran 3500w on a stock 91 chevy alt and 2 batteries for a while and i was fine... i really needed an alt at the time but i couldnt afford it if there isnt an alt available for your car you dont have any choice ... you just wont be able to play long without draining your batteries (when i say play long i mean as long as someone else who has an upgraded alternator)
in short you should be ok just as long as you dont push limits and try to fuck something up know your system and what it can handle when voltage gets low and things start getting how shut her down