Have you looked at the code available here: http://nootropicdesign.com/defusableclock/hack.html
It is not difficult to modify the code. To change the countdown duration, just change the declaration of COUNTDOWN_DURATION.
To change the pins for detonate/defuse, change these lines at the beginning of the function countdown():
// assign random pins
defusePin = random(WIRE_1, (WIRE_4+1));
detPin = defusePin;
while (detPin == defusePin) {
detPin = random(WIRE_1, (WIRE_4+1));
}
For no randomness, you could just assign them:
defusePin = WIRE_2;
detPin = WIRE_3;