I noticed this Bendix dual diaphragm brake booster is now being reproduced, not rebuilt, and the price is very attractive:
1970-1974 Dodge Challenger/ Charger MOPAR Bendix 8" Dual Power Brake Booster | eBay
1970-1974 Dodge Challenger/ Charger MOPAR Bendix 8" Dual Power Brake Booster | eBay