The Dashboard Top ( Some call it The Crash Pad ) bends and cracks with age. I suspect this is because the covering shrinks due to the sun shinning through the Windscreen. This shrinkage is common on older Cars probably due to the type of covering used ( Vinyl ? ). To remove slide something like a steel rule as in picture below between the Dash Top and the main Dash. Gently part the Dash Top from the main Dash. Take care as the less damage you do will mean less repairs. The Dash Top should come away relatively easy as the foam will be quite brittle as will the Vinyl so take care.
To try and remove the Vinyl would not be a good idea as below is a type of foam that will disintegrate.
The way I succeeded was to cut angled groves length wise in the foam from below. The angled groves were cut using a Tennon Saw, the idea is to cut the groves so they are able to allow the foam to bend ( Closing the groves ) and so the Top can become straight. The groves can be seen on the Image below with the Concrete Block. When the groves are cut and any cracks are held smooth with some tape.
Apply some Glass Fibre Resin to the Foam and groves and place some course Glass Fibre Matting over the Resin then using a brush work the Glass Matting well into the resin. Using a heavy object ( Concrete Block ) to push down on the Dashboard Top ( From the bottom of the Dash Top ) will allow it to become straight. If you place some plastic between the Concrete Block and the Dash Top it will prevent the Block becoming stuck to the Resin. Normal plastic shopping bags work well.
Due to the size of the Dash Top I done this in six stages. Take care when placing the Block on the Left side of the Dash Top as it curves up ( As viewed from below ) so you will need to have the Dash Top Right side off the end of the work bench or you will end up with the Dash Top bend from left to right. When all of the bottom of the Dash Top is completed you should have a straight Dash Top ( Nearly straight, once covered and fitted and slight imperfections will not show ).
I will not even try to give advise on covering the Dash Top. I have all the skills of a one legged arse kicker when it comes to doing this. I am pleased with my efforts, the finished Dash is not perfect but it looks nice. There were some slack bits that appeared the next day but they were easy tightened/removed. My only piece of advise is to use a covering that has some stretch to it. This will enable you to get the covering around the awkward edges, also start in the middle and work outwards. It may seem easier to glue along one edge then across but I was not able to get an unwrinkled finnish especially at the edges unless I started in the middle. Note if you are using some type of Vinyl or leather cloth make sure you warm it first and stretch it when fitting. If not, halfway through the first summer you will have more wrinkles than your grannies bum. This is important especially if you are doing this in the winter when cold.