I can confirm it - at 40 MHz heatsink is a must and you should use at least 33MHz processor if overclocking that baby!
My Q605 had processor socket with notches which permitted fitting a heatsink from Q650/700 variety. I also installed real 40 MHz 68040 and changed Motorola PLL chip. After these mods machine have been stable for something like 8 years or so.
ojfd