I want to preface this with the old adage Opinions are like ashats everybody has one and they are just opinions. As far as going original given the parameters you laid out I would say go modern on the a/c. There are 2 major brands of which I have personally not used but have been exposed to that seem to have their advantages and disadvantages so do your homework. I assume when you talk about the vintage system not working well you mean the original. they improved the 69 systems over the 7&8s but the original in perfect mechanical working condition is adequate but no where near as efficient as a modern system as far as power consumption and output. I have read complaints about I don't remember which modern system that they don't use a fresh air door, if true that would definitely not be ideal. I am an original type guy in my fuddy duddy years but not being constrained with that on a restomod frees you from the time and expense of the Easter egg hunt.