Depends on how bad the rust and crud on the surface is. I use 100-120 grit a lot when getting down to the metal. I have also used a bit knotted cable wire brush on frame and inner side of fenders.
Damage really depends on where you are sanding and how bad off what your sanding is. I have been using a D/A with 100 grit pads on my 72 which is essentially the same truck as yours.
Are you looking to get down to bare metal?
