Great job, I love it. Any chance you could do a run down of the major part #'s? I can't make 'em out in the pics.
If you click on the picture they usually expand to a much larger size. Here are the PN's just in case:
Qty Description P/N
2 ea Tachometer Can 74658-05
1 ea Oil Temp Gauge 75321-05 (could have a different PN for the kit)
1 ea Voltmeter Gauge 74552-04 (could have a different PN for the kit)
1 ea Bracket, Base, Chrome Billet 67150-04
2 ea Rubber Gauge Isolator (don't know PN for these -they came from the Electronic Compass kit I think...)
1 ea Billet Gauge Brkt, Dual 2" 67648-04
1 ea Brass Pipe Bushing, 3/8" Male Pipe X 1/8" Female Pipe -from the hardware store
Great job did you wire the volt meter all the way back to the battery or to something in the nacelle? I have a oil temp gauge (mounted differently - see photo) that I wanted for the same reason you did/do and now want to balance it out with the voltage gauge.
I'm at work and can't see the website where your pictures are posted.

Since you already have the oil temp gauge installed you could either splice into the power for that gauge, or find another switched 12V source inside the nacelle -whichever works best for your installation. It's not a good idea to wire it directly to the battery because it would run the battery down after a little while...

Good luck guys, and let me know if you have any other questions.