I seem to remember something about grinding off the wide spline or similar for cross compatibility on the Sheldon Brown website. But if it was me, I'd probably look for a freewheel wheel and a 6-speed f/w instead. Easier access to parts than 6-speed cassettes. While the inner sprockets might be easily enough dealt with, I don't know how you'd get around the fact that a 6-speed cassette has a threaded smallest sprocket that doubles as a lockring.
Or if you're doing friction shifting, go for 7/8 speed instead.
Or hunt for some used 7/8/9 indexed shifters, and do an overall update.