in this particular case the dustcap needs to match the cassette body. you need the cone/dustcap combo that goes with that era body.
I recently did something similar. I took an old 6spd uniglide wheel and put a 7spd hyperglide body on it but I had to rob the axle and cone/dustcap from a 8spd hub I had lying around.
check Loose Screws, Bike tools ect, Harris Cyclery. with the Harris Cyclery site you can access the QPB inventory.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk