The spacing between 7 and 8 is the exact same. It's the reason road bikes went from 126mm to 130mm; it accommodated the extra the gear. If it's a newer wheel then it already had a spacer to remove slop in fitting a 7sp cassette. Toss the spacer in the parts drawer, put an 8 speed cassette on with the low cog the next size bigger and enjoy having the same 7 gears. Shouldn't even need adjustment. If it's a 7 speed wheel many of the 8 speed cassettes are screwed together to make the block, buy one that is, toss the screws and the last cog and spacer and enjoy a 7 speed cassette. No friction shifting needed.