Best glass filler material I have yet worked with is Upol Fibral. It's much more spreadable than the various "kitty hair" and "tiger hair" products and sets up very hard and strong. It also sands hard so it's not something you want to overfill with.
If I were doing your job I'd make a template for the new gauges out of hard board so I could can fit it up for looks before drilling the new holes. Then I'd take the gauges out, remove the eyebrow bit and mask that area down to the line where the flat face meets it. I'd drape plastic over everything under it and then 80-grit the front of the dash, then acetone the back, then 80 the back, then acetone twice more.
Then I'd tape over the front face of the binnacle, tightly, with good masking tape, then resin the back and lay up two layers of 6 oz cloth. Let it dry, peel off the tape, 80 grit that, acetone it and fill the holes with Fibral. Don't mound them.
Let it dry an hour and acetone that, then 80 it again, acetone again and come in with Upol "Gripped" filler. Again, Bondo, Evercoat etc. are fine but the Upol stuff is worth the extra scratch, imho. Easy to spread, easy to work, no pinholes. Two spreads of that and I'd be down to 220 grit before sundown.
Next day sand it out to 400, lay in your template, mark the holes and cut them, 220-300-400 the cut edges and test fit the gauges, then spray to match and Bob's your uncle.