I made some progress on the AC. This has been rather difficult because with the new engine in there, there isn't enough clearance for even the factory non AC air box. Like the brake box on the other side, I am doing this without cutting or modifying the firewall. Depending how much spare time I have this summer I might work on a new heating system. With the other car being a convertible theres no such thing as too much heat or too much AC when you need it. I am using a mid 2000s Silverado evaporator. I will need to make a couple adjustments but overall I'm happy with how its looking.
Heres a later model heater core I was thinking about installing. At first I wanted to do a firewall mounted setup with both heater and AC but ran out of room in a hurry with just the AC so heaters staying in the factory location