You might be able to make a mask to fit around the Cutlass Supreme cluster. If it is all black and fills the RM opening it should look nice. You could use aluminum, to make the mask, and have it powdercoated in flat black. It could fit in the small recess around the original mask. It would taper in the sides, and if necessary taper out along the top and bottom. You might be able to just make the mask cover some of the excess height. You have to figure out a way to mount the donor cluster to the dash support, and keep it from sticking too far out. A sheet of fiberglass panel, or acrylic would probably work.
Another option is to replace the woodgrain with engine turned aluminum (powder coated what ever color you like), or real wood (stained to match and gloss coated), and make the cutout to fit the cluster. You could make a mask to fit the opening, and attach the cluster to the dash support. You would have to attach the replacement to the dash front. You could attach it from the back, or use stainless hex pan head screws from the front with a backing plate to trap the dash between the two. I am giving you some of the ideas I have for fixing my dash in the future.