I used to have a 1990 Bianchi Grizzly w/ Suntour XC Pro (I've since sold it to my younger brother). The 7s shifting on it was pretty good. It improved dramatically when I got a set of MTB wheels with Shimano cassette. I solved the spacing problem by taking the spacers from a Suntour cassette and using them between the Shimano cogs on the Shimano freehub body. The Suntour freehub body is a larger diameter than the Shimano body, so they went on easily. The mix of the XC Pro RD, XC Pro Accushift thumb shifters, and Shimano Hyperglide cogs shifted better than any other drivetrain I've ever ridden or worked on. Not sure where you'd find Suntour cassette spacers anymore. Were it me, I'd look into taking apart an old Suntour 7s freewheel for the spacers.