why not include a pre-built (look on niagara cycle and jenson usa) new rear wheel with freehub, so you can upgrade the gear cluster easily? there's a 7-speed shimano freehub cartridge that goes from 11t to 28t to which i upgraded and i love it. i'm rarely in 7th but when i do it's Very swift and 28t is quite adequate for climbing. i don't know that there's a similar cartridge for freewheel bodies but check ebay for 'megarange freewheel' - it's probably the closest vintage equivalent.