System 6 is not officially supported on 68040 Macintoshes, but has anyone successfully run System 6 on an early Quadra, e.g. the Quadra 700?
Quadra 700's had a ROM-version which was 32-bit-clean (not using one byte out of all 32-bit-addresses anymore to allow addressing of more than 16MB RAM). Therefore it is quite unlikely, that anyone succeeded in running any System 6.x on a Quadra 700 computer. At least this version of System 6 needs to bring its own 24-bit-memory manager in RAM with it.
Just be curious: why is System 7.0.1 no option for you?