The length difference is 1.4mm between a Shimano 6 speed and 7 speed freewheel. If you have indexed 8 speed shifters you're trying to use, a 7 speed freewheel might fit with acceptable clearance, and you can usually get an 8 speed shifter to index acceptably on a 7 speed freewheel as the cog spacing is 5.0mm 7 speed an 4.8mm on 8 speed, which over 7 gears gives you a maximum error of 1.4mm--you can average this out so your error is lower, and shifting is generally good.