If you mod a ROM, it may very well still work with an emulator, but on a real Mac, you will need to give it a new checksum or else you will get a sad mac. The checksum is stored as the first 4 bytes of the overall ROM, when it's put back together. I made a simple program that can compute a new checksum for you:

http://www.d.umn.edu/~bold0070/projects/checksum/

Run your modded ROM file through it. It will tell you that the checksum failed, and it will give you the actual checksum. Simply paste this over the first 4 bytes of the file and run it again to make sure it passes.

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On second thought, I'm not so sure that it does tell you the actual checksum! If you're not familiar with that sort of programming, let me know and I'll fix it for you.