[nUcLeArEnVoY]

What was your engine temp? I mean, going uphill at cruise speeds provides a nasty combination of max vac advance, potentially high centrifugal advance depending on your advance springs, and of course high engine load, so if anything would bring about pinging, it's that. Maybe up that fuel octane a bit? I only run REC-90 in my '79 400, but I'm pretty sure it's lower comp than your's, since I'm still using stock vacuum advance which adds 25 degrees at 11" on top of an 18 degree initial timing as per stock spec (but then again, my advance springs are EXTREMELY stiff/heavy, so I'm getting very little centrifugal). But even with all that advance, I don't get pinging... but I'm also in South Florida where hills are those things you see on Cuban girl's chests.

As has been mentioned, run a quick compression test with a good tester using schrader valves. I would think you may have to up that octane a bit. I say go back to the 89 - clearly it was your happy medium. Vac leak could also be possible, but I would think you would have noticed your idle raise or drop depending on your mixture. If the only difference between this newly onset pinging and no pinging before is that you tried a lower octane fuel, then I think that may be where you should start.