I think those 18's look good. Some do look too big in my opinion. I think it has a lot to do with stance and tire size which you guys seem to have just right. I'd prefer 17's if it were mine, but the 18's don't look too shabby either.
Seeing those pictures is like I'm getting to look into the future. That's exactly what I want my '66 GT fastback to look like when it's done. Wheels, color, stance, everything. That's the mental picture that I've had in my head since I bought it.