I took a look at the wiring diagram and data for '63 and the book says 15a fuse for deluxe heaters and 10a for recirculating heaters. That is for '63 but I can't imagine it would have changed that much by '66. I also looked at the fuse box for my '63. It says the same thing on it, Heater 10a, A/C 20a. Keep in mind they used the same fuse box regardless of heater application. So, I would follow the book and use a 15a fuse with your deluxe heater.
