+1 On threadless... It is much easier to find $200+ stems for threadless.
It also opens your options to 31.8mm bar clamps so you can get the $400 handlebar you've been waiting for an excuse to buy.
The stiffness of the 1989 frame would depend on model. I believe that they ran the breadth from Touring to Criterium geometries in 1989. MY former Cannondale Touring was a pretty smooth ride, and I believe it was a 1989... It did have a steel fork though.