Sorta. The spaceing of your 6-speed freewheel and an 8-speed cassette are just different enough that it won't shift perfectly but it will kind of work. A 7-speed, which is find-able would be better. Try to set it up so that one of the middle gears is perfect with no chain clicking at all and the misadjustment at the two ends should be manageable. The limit stops in your derailleur will blank out the two unneeded gear positions.