Snyggt jobbat SimLego 
I've been checking your progress from time to time.
SimLego:
I put 1N4148 diodes between the Arduino and solenoid drivers, just in case one of the driver transistors would break down and short, and send 40 volts back to the output pins of the Arduino. I don't know if this will help, but at least it makes me feel a bit better.
To me that sounds like a good place to use some opto-isolators.