My '88 dash has a few cracks like yours but not as bad. was going to pull it out and araldite the cracks, it comes with a skinny self mixing nozzle which should get right into the cracks. was going to mask around the crack with masking tape, apply the araldite, sand it lightly after drying and respray it. I believe thats how the pro's do it. happy to be corrected.
cheers
AdrianW