Muscle Car Research (my company) is now selling a reproduction C9ZZ-2B022-A brake booster gasket for use on 1969 and 1970 Mustangs. I reverse-engineered this part using original car observations and an original booster mounting flange. Picture and purchase details can be found on my company web site:
http://musclecarresearch.com/shop/index.php?main_page=product_info&cPath=4&products_id=33Payment can be made using PayPal (credit cards can be processed by PayPal, too), check, or money order.