The dimensions on the hubs are different, which means that the spokes you have are too long. Which sucks, cuz you're going to have to buy a whole new set of spokes, which will cost some $36, and all you'll have is a 27" steel wheel.
The relevant dimensions are the distance to the wheel center from the flanges and the distance to the center of the flange holes from the axle center.
I'd suggest downloading the full version of
Spocalc and seeing if your hubs and rims are in there. Then you'll know how long your spokes should be.
The tool to get apart a freewheel or
freehub is usually like $3.50, by the way.