1970 Duster A/C Firewall

You would need to direct that question to the aftermarket vendor/mfgr.
Here's the thing... Those who have factory a/c cars typically maintain what they have ( just live with it ). Therefore the majority of add on systems are designed fit into the non a/c builds. And there are plenty of differences in the 2 factory setups to cause conflicts.
One more point to make here, If you just must have the OEM heat a/c, or maybe you already purchased those used parts, you still don't need a firewall. All you need is a template, pattern, guide to cut away some sheet metal at the fan hole and add a few screw holes to turn your stock firewall into a a/c firewall.
There are stamping/shape differences in the 2 different 73 up firewalls but not the pre 73.
At one time there was a section of a/c firewall being shipped from one member to another to use as a changing pattern. I haven't seen mention of it in a long while. I can only speculate that todays all new R-134A systems alleviated the need for it.