Maybe someone here can help me out, because I'm confused. I thought that if the last cog was threaded on, that pointed to Uniglide..... which is definitely a cassette.
If it is a freewheel, then your most cost effective solution would be finding replacement wheel set with a 6/7 speed freewheel type hub. You can probably find one fairly cheaply that will do the job, but if you can spend the bucks for the custom build it is hard to beat Open Pro's.
If you're going to upgrade down the line to a 10 speed setup with brifter's, then RobbieTune's suggestion would be the way to go - "New 8/9/10 Shimano, 7-sp cassette, and a spacer"
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