I think you may have a USA made frame which is really nice. It looks like a great find!
You can easily convert it to seven speed by changing the freehup to a 7 speed and just squeexing it into the existing frame. Do not try to cold set it by trying to bend it wider permanently, just ease the larger cassette in. You can run 8-speed Claris STI sifters too and just have one click unised because the space between gears on 7 and 8 speed is the same. You will appreciate the extra gear for finer steps between gears. The wheels will last forever as is with 48 spokes, though a new wheelset of modern wheels would make it much lighter and snappy.
Last edited by dwmckee; 11-21-19 at 08:57 PM.