You will need a substantial ratio speed increasing gearbox to get the speed up for a standard engine driven alternator.
I see many serious problems to be overcome with this idea. Conventional alternators are not inclined to excite much below nominal speed so you may have to be nearly up to your pitch control speed before anything happens. For a constant load resistor you will most likely have to hold load off until high winds when the thing is already in pitch control or the load will either stall the blades or more likely throw off the alternator excitation unless it has AVR control.
The end result is going to be pretty dreadful. To even stand a chance of making something respectable you will need quite a sophisticated controller with pilot exciter and some form of approximating the cube law load. Unless you are into power electronics then the scheme is pretty much a dead duck.
If you have the ability to do the blade pitch control then you would be far wiser to use a low speed alternator with permanent magnets and get a decent efficiency rather than have gearbox losses,alternator losses and a very complicated control scheme. Even with a pm low speed alternator you will not get very good results into a fixed load but the scheme may work with series capacitors or a 2 step load change. It will be significantly easier and safer than a furling machine if you have a pitch controlled limit on speed but for good power extraction you should do all the energy capture below pitch control operating point and just bring that in at the alternator power limit.
Flux