Older frames were designed for larger diamater wheels and bigger tires than are typical for road bikes today. Switching to 700c wheels will lower your bottom bracket (4 mm if you use the same tire width), slighly altering the handling characteristics of the bike and increasing you're chances of pedal strike when cornering. Pedal strike is less of a problem with a freewheel, but it is a serious concern if you go fixed becaue you can't coast around corners. Using narrow tires (23 or 25 mm) will lower your bottom bracket even more. If you go with 700c wheels, I would use a larger tire to keep the bottom bracket at its intended height. Use at least a 28mm tire with a freewheel and at least a 32mm tire with a fixed gear.