Here are some images of Mavic GL330 rims with Vittoria tubular tires on the Cannondale. They are shown with Veloce dual pivot,short reach brakes, with recessed nuts.

In the '84 Cannondale catalog, the SR series is shown with recessed nuts and the ST with external brake mounting nuts.

As you can see, there is not much room for fat tires and fenders.

Looks like this frame is, indeed, something special.

Thanks again, for your input.