I've fit nominal 32mm tires that measured 27.0mm wide on MA40 rims (new Vittoria Corsas). Cannondale SRs tend to be pretty generous for a road bike, so fitting actual 28mm wide tires is a possibility for me, though as always, test it out yourself if you have a few sets of tires to play with.
If you check on the underside of one of the chainstays, you should be able to see a serial number.
You can decode it here. From looking at the catalogs, and your very helpful photos, I'd say yours is a 1990 SR300 with the Suntour Accushift/Blaze 7-speed and the the steel fork. Cannondale played a bit fast and loose with colors, decal fonts, and forks during this year and the one surrounding it, at least from my limited experience with them. 1990 ST400 should have had a traditional steel fork, but had a unicrown, like your SR. My current 1989 SR has a Black Lightning paint job and decal color (gold), but the fork is aluminum and not steel like the Black Lightning had. Oh well!
Regardless of rank, your SR looks super sharp and will look killer with any modern componentry you put on it.