An alternator puts out a frequency that is proportional to the rotating speed. As your car changes RPMs, then the frequency also changes, but this is irellevant to the car, since its being rectified regardless. In your case, you will not be able to reliably get 60Hz at all, as the frequency will be significantly higher and of a variable nature.
Secondly, an alternator puts out maximum amperage when its field voltage is Zero. This is a fail safe for a faded battery.
Thirdly, this is generally not going to work very well, I would say that it would be easier to buy something that is meant for the job, instead of hacking this, which won't work right in the best case.