From what I know, because factory gauges are not inclosed there really do not have a normal size but if your going customI have found that 2" or 2 1/16" fit in the four small holes and 3 3/8" fit in the two big holes. That is if you want to have them to fit more in the holes (like I did here:
http://forum.73-87chevytrucks.com/smforum/index.php/topic,24709.msg206096.html#msg206096)
However you can get two 5" if you what but they would have to be install on the front of the factory bezel so the autometer gauage bezel will be protuding out from the factory bezel. It all dpends on how you are cutting and mounting the gauges dpends on the size. My new build I have all 2 1/16" and two 5" and I am custom mounting them so they will still be sunk in and look a little factory still cause I am using the factory bezel.
-In an 85 all the gauges are electric except the speedo that uses a mechinical cable, at least my 86 was the all electric except speedo.
-Yes the custom gauges install the same they just have custom colors so they stand out from others.
-And as for the dome or flat, depends on what angle you mount them and if there will be alot of sun light in the truck or tint will help block some as the it dpends on the reflection. I usually stick with flat glass becuase it looks more like the original, just my $.02
And just remember that customizing requires either alot of trials and templetes...or alot of extra parts so you can cut them up in different ways. My first bezel I cut up to do the guages I cut them bigger and made a gauge mounting plate under it....but to me look like crap, so then I vut small holes and mount them on top of factory bezel but didnt like that either, so then cut a little bigger so then would fit down in the factory cups and be flush with the bezel. So just have patiance and find out what you like, my $.02.
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