i bought an old Schwinn Duo sport tandem bicycle. It comes with a 5 speed rear end. Would like to have a few more gears but don't know what would be involved. The bike has been changed a bit since new in the 80's; the handlebars are now upright and have a 6 speed shifter it seems.

What would be the problems involved? We would like a new rear wheel with a cassette on it. Could we just spread the stays and pop the wheel in? What other mods should i consider to make it work?

thanks

Duo