It is a little more complicated than first blush.
Matching blades to an alternator means just that matching... but all alternator designs are different..
In the case of the F&p... no matter what blades you put on them, they will NOT burn up, or even get excessively hot.. short circuited for days at a time ..It is not possible.
It is pointless to ponder betz limits and think that even in a perfect world, your blades can deliver it...the problem becomes how the alternator itself will load them at those rpm... can it load the torque you envisage at those rpm . It is beyond complicated.
In the F&P case, it will be how you fiddle with the windings that will give you your max output.
It is a composite of how many coils can be paralleled to get some copper cross section into the picture, and just when you calculate that you have done very well, the inductive reactance and leakage from long salient pole alternators will put an upper limit on your output.
If you calculate on the 200-500 watt mark, you will be in the ball park. At that point or thereabouts it will current limit, no matter how fast you spin it, it will just run away at 3 or 400 watts etc wiring design dependent.
You can use capacitors to ameliorate the reactance to some extent, and if you really get technical with them I have seen folks get 1kw out of a single stator... but that was complicated with voltage doubling and tripling and a big cap bank.
So alternators... particularly those with not much magnet, and lots of steel involved with the magnetic paths, will self current limit, those with strong magnets and air paths are very much less influenced by back MMF problems, and tend to burn up instead if over powered.
In short, you can maybe get it up to 500w if you rewire carefully, 300 more typical, but you will not burn it up... ( and 1kw if you really know your stuff).
matching blades to an alternator is not just wind speed and blade design with a calculator... the humble alternator will also dictate what works well, and what does not.
Also, some older F&P cog unless you decog them... if you don't then you will need wide chords to start them in light winds.
Your best information will come from
http://www.thebackshed.com/windmill/Contents.asp#3 It is dedicated to these things
...............oztules