I had good luck using Stiction E-Z Weld Plastic Epoxy.
I used it on an 87 GT dash. One corner was completely cracked out. I dug out the foam, and filled it with the epoxy, then used a piece of leather to grain the epoxy. It held great, and looked good. Only problem was when I bumped the corner recently. Epoxy held, but a new crack started next to the epoxy.
I cant find the epoxy in any stores anymore. Looks like it's not being made anymore?
I used a liquid plastic epoxy recently to fix another dash. It looks perfect, but I havent had it "fixed" long enough to make sure it will last