Which AC/heat system and why?

If you have a heat only car go with classic auto. That’s what I have. It’s almost a direct replacement. If you have an A/C car you can do either. With the A/C car you will have to fab a block off plate. There is a member here that has the stencil for it. I preferred classic auto, vintage air is about 4 miles from my house and found them rather difficult to deal with in person. I don’t know the firewall difference between 67 and 73 if there is any.