I've gotten Shimano 6- and 7-speed drivetrains to work with Suntour freewheels. So I don't see why the reverse wouldn't work. There's usually enough "slop" in the indexing to handle being a few tenths of a millimeter off. If you're really adventurous, you could always grind down the three spacers between the biggest cogs by 0.2mm and then the cassette spacing will match a Suntour freewheel.