I just tried for several hours today to get the Mac Garden SimCity2000 .sit to unstuff, with your trouble. It turns out not all the Mac Garden files are busted like this - consider Oregon Trail. But a surprisingly large number of them were.

I happen to have the original installers on floppy. Was really hoping I wouldn't have to bend over backwards to get the data off of them; no longer have the mac they're supposed to run on. Is there a different site than Mac Garden to be going to?