If you can find an MC68040RC40A with a mask of L88M then it will run almost cold at 40MHz (compared to the original ones which could melt diamond at 25MHz)
The history of the 68040:
(backed up with Motorola / Freescale data)*In 1989 it went into production as the XC68040RCxxyy originally a 0.8um masks (
D43B, D50D, D98D, E31F) part. These run exceptionally hot even at 25MHz and need heatsinks all the time.
*
In 1995 the 68040 qualified to mc status. It was shrunk to a 0.65um mask (
E42k) these parts run hot at 25MHz but
dont need heatsinks, famous in MAC circles for overclocking. There are some XC 0.65um parts from 1992-95 with the mask
E26A but I am unsure as to their heat dissapation or overclockability.
*
In 2000 the 68040 was shrunk to a 0.57um mask (
K63H) these run a lot cooler that all previous parts and again do not need heatsinks at 25MHz+.
*
Finally in 2002 the last MC68040RCxxA mask was made (
L88M) production was moved back to MOS11 FAB, again at 0.57um, with increased reliability (which should favour overclocking even more).