This is probably a stupid question, but if I have a 1985 era wheelset, with shimano 600 hubs, and a six speed hyperglide freewheel, can I just replace it with a 7 speed speed freewheel? I am pretty sure the answer is yes, as long as the threads are the same.

And if so, what brand/type freewheel should I buy if I want compatibility with Shimano index shifting? What are my options? Most freewheels (at least those on ebay) don't really specify the spacing between gears, and it seems like that would be the critical feature.