Challenger

8mm for the mounting bolt. I break it loose with an allen wrench and small cheater pipe. I cut a 1/2" piece off of the allen wrench, put that piece in a ratchet wrench and use them to remove the bolt. Then reverse order to reinstall the bolt.

When you buy your oring kit ask the Mack parts guy to print out the rear top cover illustration page. You can use this as a guide for reassembling. Or just mark every piece in order to help you while reassembling. Its not that difficult, but sometimes I have to break out the book because I forget which way parts face. Time seems to be important for you, so every bit helps.
Once you have the cover off, there is an interlock pin inbetween the cylinders. This allows only one cylinder to engage. The pin has to hold one cylinder shaft for the other to come out. So if you cant pull a shaft out, its because the other shaft isnt in the correct position. Just a heads up so you can save time.

You could just add a manual gauge. You would need a gauge, 1/4" airline and a fitting to put in the intake. Your intake has a small pipe plug ( I think its 1/8 pipe thread). Remove the plug, add a fitting (90 degree, 45, or straight, whichever would work the best for you), install a manual gauge in your dash, and run the airline from the fitting to the gauge. Its that simple.