If u don't want cooling issues then you have only 2 choices:
68030 50Mhz slow but cool.
68060 50Mhz superfast but supercool. No heatsink needed. No fan needed. Only 4.9 watts supergreen CPU. Save the Earth! Use an 060! You also get a free FPU and free L1 cache!
All 68040 cards are hot and must have a heatsink or fan or something.