you do have to keep the chain tight though, or it likes to jump off. maybe a spring loaded idler is better.
here is how I got two freewheel hubs on there, you might want to grind off a bit of the parts that might rub on the freewheels, mine have self machined pretty well though.
basically figure out how much to remove to center the hubs on the threads, then carefully removed it by giving the wheel a good spin and grabbing the angle grinder.
Also, you want the sprockets to be as close as possible, ie the big one is on the outside of the freewheel and the little one is on the inside, and they both have tapered countersunk bolts holding them in.