73 duster heater motor to firewall

Hello everyone, I’m having trouble with a 73 Plymouth duster getting the heater motor to mount to the firewall. I know on the earlier 70-72 they had a 4 hole mounting plate that mounted them to the firewall but the 73 is different. It has 3 studs. Someone told me that they didn’t have a seal plate that mounted the motor to the firewall. So if that’s the case, then how does it mount to the firewall? Some detailed pictures of one of y’all’s dusters would be helpful. Thanks in advance.