Im not possitive but have you tried Detriot muscle.........I know they make and sell gaskets and sets. I got a kit from them for my 67 dart...Firewall kit. Nice stuff.
Another vote for DMT — top quality, good prices, fast shipping. Their online catalogue is out of date; they have (but don't show) the '63 and '64 Dart gaskets, I bought them for my '64 in 2009. By buying them from DMT (or from Gary Goers) you support a vendor with a long track record of good customer service and Mopar community citizenship. That's not always the case when you buy from some vendors who advertise this product.