Need advice for upgrading wheel on old bike

[hondo]

I have a late 70's or early 80's 12 spd Centurion Sport Deluxe. I love this bike but I have recently egged out the rear wheel. I want to upgrade the rear wheel for something lighter and stronger.
I want to keep my existing 6 speed freewheel and shifting assembly. Will this be compatible with new aftermarket wheelsets? What modifications will I need to make? Can I just add a spacer?

[Alex Ramon]

You'll need to get a wheelset that is compatible with your 6-speed freewheel. They do still make them... you can get a 27" wheelset here.