I've got a Metzeler ME22 on the front of my 400F and a Metzeler ME77 on the rear. WAAAAAY wider than stock, but they do OK. When time comes to replace, I'm thinking of the Avon Roadrunner, which come narrow enough for the old bikes. It's a tubeless design, but American Moto Tire recommended it for the narrow rims we have, and it will accept tubes. Mounted a set on the GF's 360, which runs the same size tires as the 400F and am VERY impressed with them.
For the 750, I don't think you can beat the Dunlop K81/TT100.