If your preban colt lower is infact a pre 1994 pre ban. you can do what ever you want with it.
Doesn't matter when any of the parts were made. the parts could be made yesterday. You need to know when the lower was manufactured.
I bought a complete, surplus M16A1 upper and mated it to a late pre94 ban Colt Sporter lower with a small pin and full mag fence. I wanted a rifle that looked as close to an actual M16A1 as possible.