I have a MF-7400 6-speed and MF-6208 6-speed. Originally they were both 13-21. That was fine for the terrain where I live when I was in my 20's and 30's but I'm now in my 50's. I hate to toss perfectly good freewheels so I changed them to 13-26 and 13-30 by buying loose cogs at
http://www.loosescrews.com/index.cgi...d=908719015952 . Looks like they still have a 26 but not a 28 or 30.
You can get a 13-28 6-speed freewheel which is Shimano compatible at IRD -
http://www.interlocracing.com/freewheels_steel.html