Cable sizes (I assume you're talking about the inner cable itself, not the housing) have been standardized since before your 1981 frame was made, but brake cables (1.6mm) are larger diameter than derailleur cables (1.2mm). Your LBS should have cable end crimps capable of accommodating various cable diameters up to 2.0mm.

http://www.jensonusa.com/store/sub/1...ng.aspx?s=1590