We've purchased and received a dash cap for our '76 El Camino. It fits perfectly. Our only concern is the cracks in the original dash board, and what we need to do to them to prevent the new dash cap from cracking in the same places, because it's very thin. Has anyone else capped their dash, and what did you do to your cracks, if anything? Would taping them help?

I would give serious consideration to using silicone gel sealant. It will remain somewhat flexible for the long term and I would think that is what you need.

I installed a dash cap on my '77 Elky Classic a few months ago and only smoothed out the original dash by trimming off the high spots on the cracks. This basically made the cracks wider but when I was finished, no high spots remained. I didn't fill the cracks with anything but one guy recommended filling them with some kind of cement (sorry I can't remember exactly what he said!). Following the instructions that came with the dash, I put a quarter size dot of silicone (that came with the dash cover) about 8" - 12" apart. Although the dash doesn't just fit snug and wouldn't pass a concours restoration, it certainly looks light years better than it did.
I didn't take any before or during pictures of the installation :-( but did get some after pictures:
Dash Del Camino Enrique (http://elcaminocentral.com/users/ponchovilla/DashPictures.html)
Good luck!