Wow, lots more information than I expected, thanks.

I realize looking back that my original post wasn't very well worded or clear. I apologize as I am new to this and probably am not using the right terminology.

To clarify, I am not trying to replace the cogs on the current freewheel. I just want to swap out for an entirely new freewheel but wasn't sure if a late 80's vintage Shimano 7400 or similar 7 speed freewheel/cogs would work in place of the 6 speed Regina freewheel I have now.

The only reason I am concerned about the chain is that I recently bought a NOS Everest "Serie Special" chain that is correct for the vintage of the bike and I'd kind of like to use it. It is supposedly a pretty nice chain, and very light with side plates/links that are drilled/cut out. All is says on the side of the box is 3/32 x 1/2, and I'm not sure how to tell if it is narrow enough to work on a 7 speed Shimano Dura Ace Freewheel from 1989.