IMO, a minimum of 235/60/15 for the rear.I do not think they look gawdy or pathetic.If anything, the original sized tires look too small for the car.IMO,if you put two identical cars,one with original 14" rally rims and tire sizes next to one with later 15" rallys and the wider tires and the car will have a much more appealing look. I currently have 245/60/15 and they look nice and aggressive.I've had the 275/60/15's on my bird and they make the car look much more aggressive.Anything larger than that and it's a bit overkill for a street car.I personally like the black walls facing out.
I have 245/60/15 Dunlops on my 69 cutlass and they look good with the white letters but i'd rather to see them with the black walls out.
We all have our own personal tastes,good thing we all fit an FGF into the equation

David