If you don't mind driving up to Buckhead, I like Buckhead Imports at 28 Irby Ave (across from Henri's bakery) 404-816-9606. They have been doing the major mechanical work on my two for years. They also service Mercedes and Jaguar and it is not uncommon to see E-types getting service. The shop is owned by Jason now who bought it from Lloyd. Lloyd still works three or four days a week and he has been repairing BMW's since our 2002's were new. Everyone there is nice and helpful.
If you don't want to go that far north, check out RWL Imports at 520 Edgewood, just east of Boulevard. 404-588-1624. Bob services Porsche, BMW, and Mercedes and there are usually older 911's and 356's floating around along with 80's BMW's. I have never had my cars there, but they have come recommended to me by several sources and seem to have a very good reputation in Atlanta for servicing older German autos.
Hope this helps,
Aaron