I've been playing with dashes way more than I care to admit lately... ANY dash swap is tougher than you think. The bigger the difference, the more nightmarish it gets.
Here's a fine point: The mount posts on late guages are threaded metric. pre-77 are all standard. Not that it matters, so long as you have the right gnurled nuts, it's just that there's A TON of little things like that. Different lights, different light sockets, differnt socket mounting systems. Different harnesses going to the whole shmear. On and on it goes.
And now for something completely goofy... I'm seriously considering making a full carbon fiber dash. I know I can, and I know how I'd do it, I'm just trying to decide if it's a completely dumb use of time and resources.