10speed

The bike is a mid 1970's Mitzutani, with a Suntour rear derailleur. 27" wheels. The wheels don't look very exotic, Araya brand steel wheels. I am thinking that if I look around, I can find the same size wheel with 6 sprockets on it and that the derailleur ought to be able to be adjusted...

Is the fastest gear on a 1970's era 5-speed the same as the fastest gear on a comparable 10-speed, or is the 10-speed essentially a 5-speed but with an overdrive? I am talking for mainstream bikes of the period such as Schwinn's and Huffy's.

A friend of mine uses a 9-speed chain on his 10-speed Campy Record bike. A 9-speed chain should last longer and is cheaper than a 10-speed chain, so I'm wondering if it's possible to do the same on my bike. I'm worried about the width of the chain and whether it can smoothly move from cog to...