Hi Dave,
I have a 1966 Atlas, and I've always run an Avon Roadrider Universal. Currently on the bike is a 110/90 - 18, but I can tell you that the side to side clearance on the swingarm is tight. Very doable, but it's tight. Avon lists the width of that tire as 116, so it's sort of funny that it's marked 110.
None the less, the tires in my opinion are great. Even for very spirited riding, I've always had great traction with them, and would highly recommend them. Of course, I'm just a normal guy that rides on the road. But I've also seen then used extensively on the track as well.
I'm in the process of changing to a 19" rear wheel, simply because I prefer that set up. But I'll be running another Avon on that as well.
I'm sure that you'll get several opinions, so like many things, it's a lot of personal preference. I honestly feel that very few people riding vintage road bikes could out-ride any modern tire. So my recommendation would be to go with what you like, and what you feel looks appropriate.