I ran through the shifts with no chain and the 7 speed wheel in... and it did not line up. first and last were exactly lined up, though...
Not sure how that can be since 7-spd cluster is about 3.5mm wider than 6-spd.
Yes, 6-spd Hyperglide was definitely made, started in 1988 I think. You can even replace the spacers on the 7-spd cluster and toss out the biggest cog to make a 6-spd from 7-spd.