In answer to the original question, I'd use a 7-9 speed 3/32" chain on a 7-speed cassette. You can check your sprocket thickness at http://www.sheldonbrown.com/cribsheet-spacing.shtml. As long as your chain is wider than the sprockets, you'll be fine. Some 8-,9-, 10-, or 11- speed won't fit on some 7-speed cassettes.

In your situation, however, I'd respace the frame and use the pile of nice new components. Alternatively, I'd flip the new components and get some retro hardware for my retro frame.