I'm piecing my a/c system back together as well. The only parts I didn't change out are the evaporator and the expansion valve and, of course, the evaporator had a leak. I ordered new ones from "Classic Muscle" in South Carolina. The evaporator was 260 + shipping. They had it in stock and it shipped out the next day.
I received it yesterday and it looks pretty good. I'll be installing it this weekend. I'll let you know if there are any problems.
Kind of hard to navigate their site but click on "Chevelles", then "Order Parts", then enter part number 3016987 and click "Search" It will show you a photo of the part.
It didn't come with o-rings so you'll have to order some or get them at the parts store.