There's a bunch going on with a wheel and changing one variable can change a number of aspects of the equation. With a freewheel hub, going from 5 speed to standard 6 speed will increase the unsupported length of the axle on the drive side and maybe require adding dish to the wheel. Going to 7 speeds increases the "problems". Of course I'm not a small person, YMMV....

Now I'm sure Mr.WGB has a plan, but for my Clydesdale purposes, I would be thinking about running 5 speeds on a 126mm spacing, and reducing the wheel dish. I've got a couple bikes that came set up this way from "the factory"