I'm looking at upgrading the rims on my Schwinn Traveler to a set of alloy 27"ers that are 126mm spaced. I know that I will need to spead the rear triangle of the schwinn slightly to make the fit and that's not really a problem. What I'd like to know is if I'm going to have a problem with the current Shimano 400 series derailleur that is currently on the bike. I've tried to do some google searching but haven't turned up any resources that will tell me the tooth or cog number ability of this RD. I'm looking to put on a 6 or 7 speed freewheel with a 12-28t.

Anyone have any ideas? Thanks!