Actually working on this right now, because I own a '68 and an aftermarket bezel is $$. Oh, and as all '68 owners know, the universe hates us and we can't have nice things
I did this so far:
1.) Disassemble the dash down to just the black bezel
2.) Make a cardboard template of the old speedometer, marking where the bolts go through and keeping the two "alignment pin" locations.
3.) Transfer the template to some MDF
4.) Cut out paper circles to match the new gauge dimensions. Specifically the outer bezels
(not the hole size).
5.) Cut the "old speedometer" opening larger by cutting back the plastic on the sides and grinding down the letters (where it used to say wiper, etc).
6.) Arranged the gauges, cut reliefs for the speedo and tach in the plastic (as the new gauges are bigger in diameter than the old opening is tall) and marked their centers for hole-sawing.
7.) Harbor freight hole saw kit! 3 1/2" and 2 1/8" seem to work.
Now I'm at the finishing stages, where I have everything in filler primer and I'm going to start doing final modifications for a modern radio, etc.
I'll see about getting pics. Biggest thing (at least for a '68) is that it looks like the max width you have for a speedo, tach, and 4 smaller gauges is 16.5", which means I'm going to have to go with Speedhut and not Autometer due to bezel size. Autometer's bezels are bigger and I don't have ANY room to spare.