Difficult to visualise but after making good the dashboard could you attach a piece of wood to the back to allow plenty of meat for screws to bite into without splitting dashboard?
Richard, that method would move the dash panel back and there would be interference with the Lift-The-Dot pins for the toneau cover. It would also make it necessary to remove the windscreen posts to get the dash out in the future if necessary. I've pretty much decided to go the proper route and move the posts forward, where they will fasten solidly into the scuttle frame. I'm having the bonnet and cowl painted, so I'll just have the shop fill in the holes and blend in the repair.