Mega2560 won't do 8 MHz 3.3V, needs 4.5V:
ATmega2560/ATmega2561:
• 0 - 16MHz @ 4.5V - 5.5V
Need a 2560V to get that slow:
ATmega2560V/ATmega2561V:
• 0 - 2MHz @ 1.8V - 5.5V, 0 - 8MHz @ 2.7V - 5.5V
'1284P will do 8 MHz, 3.3V:
0 - 10MHz @ 2.7 - 5.5V