Hi Sweetwind.
Sorry to give you bad news so close to Christmas but you'll never see 500 watts from any car alternator no matter what magnets or wire size mods you make at 150 rpm.
I would say the best you'll get is 5 watts. 500 watts at 150 rpm is going to be a much much larger machine.
Check my stories and diaries for all the wind tunnel and truck mounted tests I've done over the years on many difierant modifide car alternators.
I've seen 200 watts from a car alt at 30 mph around 1500 rpm.
Jerry