In the May issue of C't Magazine was an article about upgrading 040 models
of apple. They simply change crystals and add a fan.
Hammerhead: Centris 610 was able to work with 25 MHz (cooler prefered)
            and for 1000 DM you get an 68040 (33MHz) which works with the
            appropriate crystal at full speed. (extra cooling required)    
            Centris 650 like Quadra 700 with extra cooling 33MHz works on
            most machines and for real power enthusiasts they used a
            Quadra 950 at 40 MHz wow! But for better description you should
            get this issue of C't (a german PC magazine!) 

Christian Bauer