As far as the firewall goes, the heater hoses are in a different spot as well as the heater box mounting holes. I would try and find someone within driving distance of you that is parting out or scraping a non A/C A-Body and either get a cardboard template of the firewall or see if you can cut it out. You can use it to get what you need to make yours look correct, you may be able to get the whole box, switches and cables from the same car.
