Jack,
Since our dash is fiberglass and the small gages are the same size (unless you've got some weird ones), I would use a sanding drum or roll and enlarge the holes by hand drill or die grinder. It's only the difference between 3 5/8" and 3 7/8" diameter for the speedo and tach.
Sanding drums
I've never used one of these, but it would probably work.
Flap sanding wheel