I just went through tht on a conversion here.
Bought the alternator from Napa but it had a 7/16" pully. I needed one for a half inch.
They ordered a 1/2" pulley for me. I think it was about $10.
Bring your alternator along and they should be able to fix you up.
Another way, if you have access to a lathe is to cut your 1/2" pulley wider.