The problem here is that the term "Musclebikes" wasn't coined until the late 1970's when all the Jap manufacturers had a 1000CC plus "Sportsbike". (By then, Honda had the CBX as it's big sportsbike)
Up until then, the the CB750 and Z1 900 were referred to as "Superbikes", but when the big 1000CC (+) Honda, Suzy and Yam's were introduced, magazines around the world started referring to big jap bikes as "The latest crop of Musclebikes", etc. The 'Wing was always referred to as a "Tourer".
As far as the GL1000's straight line speed goes, well, I had one and it was plenty fast, but very heavy, and the Z1 and even the Suzuki GT750 (also referred to in the January 1972 edition of "Motor Cyclist" as a "Tourer") could kick it's butt in a straight line drag. As I said, great touring bike, but at almost 650 pounds, it had no sporting pretensions, and it'd be a stretch of the imagination to accept it as a "musclebike" in the same company as the Z1 etc.
As it was summed up in the May, 1975 edition of "Motor Cyclist" magazine, "We see the 'Wing as a far-out extension of Honda, the massive, sophisticated Grand Tourer of the late Twentieth Century". Prophetic words, indeed. Cheers, Terry.