Got it! I cleaned both surfaces again very thoroughly, and, upon the recommendation in a PM exchange with Chudson (I guess NASCAR and F1 can coexist after all...), corroborated by my machinist, I used a bit of vaseline on the o-ring, and then pressed the two pieces carefully together by hand, rather than in a vise as the manual states. Voila! I didn't even have to struggle - it popped together with no difficulty. I then used the alignment tool from Paragon to align the body of the oil/water heat exchanger to the block, and the OPRV slipped into its slot perfectly. The tool wasn't cheap, but it really made the job much easier, so I would definitely recommend using it, as the OPRV flexes quite a bit where the two pieces come together, making it not the ideal device for aligning the cover. In the unlikely event that anybody else is going to do this job in the near future, let me know, and I'd be happy to lend you the tool. Here's a picture, with my purdy new Canton oil filter:
By the way, I ran that test to see if the o-ring expands or contracts when heated, and I couldn't detect any change.