If you want the bike to handle like a new one over the life of the tire, get the same size as original or as close as you can.
All new tires will handle well when using its as-new crown radius. It's keeping that crown radius over time and use that the handling changes. Stock sized tires are MUCH better at keeping that tread crown radius.
Retail businesses want to keep their stock flowing, and there is really not a volume call for the old sizes, so no stock. Especially when they can sell you a conversion to one that they have in stock. I've yet to find a tire dealer that will refuse to special order a tire for you that they don't have in stock, though.
So, look at the manufacturer catalog, rather than the tire dealer catalog. If a tire manufacturer offers the proper size, a dealer can get it, if you can budge them off their ass enough to stop making persuasive arguments that have worked well for them in the past.
I couldn't get the AVON site to work on my old computer. See if they even make inch size tires. They simply may not care enough about your antique to make proper tires for it.
However, I know that Metzeler still offers a 3.25 x 19 Lasertec for the front, and an ME-77 in 4.00 x 18 for the rear. I get this combo for my CB550s and 750s. And, they wear evenly and keep their tread crowns shape with normal wear.
Oversize tires usually require higher tire pressures to keep the sidewalls stiff enough to prevent tread cap shifting sideways on the rim (makes the rear of bike feel squirmy). This also makes the tread crown puff up so that you get a flat spot across the tread. Then when leaned over, it climbs the tread toward sidewall, forcing you to push the bars in counter steer to hold your turn line.
Then there is the aspect ratio difference between metric and inch size. 3.25 equates to 82.55mm. So, a 90 size tire is what the dealer will sell you (or even bigger to make more profit on an "up grade"). But, they aren't riding your bike, you are. Tire dealers aren't much different than used car salesmen. Full support to the end of the driveway.
A 3.25 is 95 to 100% of that tread width tall. A 90 metric is only 90% of that tread width tall, or 81mm. A shorter front tire will quicken the steering. Sounds good, unless you prefer neutral steering without fighting the handle bars in every turn.
Bear in mind that the 550 in stock form is so neutral in handling, that it will steer reliably with hands off bars by simply leaning in the direction you wish to turn, and then straighten right up by shifting your body weight back to center. Some of this is lost with Metric tire fitment, and a lot of it is lost with oversize tires. All of it is lost with worn oversize metric tires.
Cheers,
