There is a picture that might help you here:
http://www.scaldinghot.com/gaugepage.htm If you are using the 5 inch ones, they should fit snugly in the factory bezel (I used to have the same setup in my car). They used to come with a U shaped piece of metal that you could shorten and hold it into the bezel. I had to get a replacement gauge and they sent a round mounting bracket that would not work. You could get some 1/2 inch metal bracket and bend it and modify it to secure it in place. Send me your email and I will try to send you some better pictures (they are on my home computer).
I have seen others that mount the gauges from behind the bezel. You have to slightly modify (so that the new guage bezels will fit inside the factory figure 8 piece and then that mounts to the back side of the factory bezel) the figure 8 shaped bracket that was origianlly used.