If I understand what you have done and are trying to do, the problem is: a) that 8.1 will not run natively on the 550, and B) that, to my knowledge, you cannot use the ResEdit hack with the 550 logic board. Both work with the 575 logic board, but you get a double whammy when you try to use the 550 logic board.
The ResEdit hack does not give you 640x480 resolution on a CC/575 Mystic, but rather, only allows the 575 logic board to run the machine in the CC's native 512x384 resolution. The 550 logic board, on the other hand, will run it at that resolution without any hacking, as the 550 logic board is for all intents and purposes identical with the one a Color Classic II. (There is, as I understand it, one small difference, in that the 550 has 256k more vram than a CCII board, but this does not seem to affect the way the board is recognized in the machine.)
To use the 550 logic board, and assuming it and the other internals of the CC are working, you will need merely to install a fresh and unmodified system 7.x .
The ResEdit hack does not give you 640x480 resolution on a CC/575 Mystic, but rather, only allows the 575 logic board to run the machine in the CC's native 512x384 resolution. The 550 logic board, on the other hand, will run it at that resolution without any hacking, as the 550 logic board is for all intents and purposes identical with the one a Color Classic II. (There is, as I understand it, one small difference, in that the 550 has 256k more vram than a CCII board, but this does not seem to affect the way the board is recognized in the machine.)
To use the 550 logic board, and assuming it and the other internals of the CC are working, you will need merely to install a fresh and unmodified system 7.x .