I will be doing this with my 65 C20. I know there has to be some rust from a leaky gasket, and the original owner made the dash look like swiss cheese with all of the screw holes to mount who knows what. I have a 65 AC dash sitting in the backyard that needs to be removed from the portion of cab it's attached to. Removing that dash will be my practice for removing the other swiss cheese dash. Not looking forward to it, as I have read and watched videos of dash removal, but in the spirit of having more access and repairing rust and replacing items under the dash like a two-speed wiper motor, and also the windshield, it will be worth it.
Unless there's some serious rust holes, I wouldn't remove the whole thing. Drilled holes can easily be welded up and dressed smooth prior to painting.
I don't think R&R of a dash would be easier than just working underneath it on things like a wiper motor. This is coming from someone who did quite a bit of sheet metal replacement.
But do what you must.
