How hard are these to change, really? In the factory manual it looks like, and described as, generally pulling the tubes through the heads, changing the seals, and re-setting them back. Is it really that easy?

it's easy. just make sure to gap everything properly and put everything back in the same location.

Yes, they are easy to do. Just do one at a time. Re-torque after each one! Clkark's Corvair has some great how-tos. Check out their site. I used the viton "O" rings and some silicone lube to install. Runs great. Let us know how you make out!