The subject bicycle sounds like a circa 1978 Miele Uno. It used Tange #5 (plain gauge CrMo) main tubes with hi-tensile stays and forks. In 1986 it had been full hi-tensile and in 1987 was upgraded to a 700C wheelset. It was the bottom of the line sport model. A 700C conversion is possible, though you may also require longer reach calipers.