I am not aware of block off plates for B body's being used by the factory. When I removed the heater box in my '69 RR (non A/C) I used the following:
(2) 2094985 rubber plugs for the heater hose openings.
(3) 2094984 plastic plugs for the heater box mounting studs.
This was done a while ago so the numbers may have been superseded or even discontinued. Also, as I stated my car does not have A/C, so I would imagine your car probably has more holes in the firewall.