I always find it easier to stretch the seal a little bit while you're installing it so it fits the channel better. I've also learned to order two pair of seals whenever I need to replace them, just in case I mess the first set up.
IIRC, the book says to chamfer the bottom inboard corners of the seals so they don't pinch.
Make sure you use plenty of lubricant along with some nice, smooth guide blocks when you install the cap or you'll tear the seals.
--Mark
1973 SIII 109 RHD 2.5NA Diesel
0-54mph in just under 11.5 minutes
(9.7 minutes now that she's a 3-door).