I have just fitted a set of Dunlop K82s to my 1959 Triumph 5TA Speed Twin, of the sizes you mention. They have lots of tread and look the part. They seem to be one of the very few choices of matching tyres in those sizes.
Within the speed capabilities of my twin, the handling and grip seem fine. Can't tell you about service life, mine have only done about 1000 miles.
The diameter of newer tyres will still be 18", but with car radial tyres the 3.25" and 3.5" would be expressed in millimetres. I am unsure whether metric units are used on any motorbike tyres because I've never looked! I'm sure someone will tell you.