A QUICK QUESTION ABOUT MANIFOLD VACUUM

Well that rules out chain stretch, flyweight/spring chatter, and VA flutter.

With that 262 cam, the intake vacuum should be rock-solid by 1800>2000rpm. The cam has settled down, and all the intake air should be going in the same direction. The manifold vacuum, should have increased steadily from idle, and peaked in that pocket, and it should NOT be fluttering around 1700.
If the timing is solid, then it has to be; a bad plug, rotor phasing, induction X-fire, the feint beginning of pre-ignition; or maybe, AFR but that, I doubt.
To help in diagnosis, I would check the spark-strobe of every wire, beginning with numbers 5 and 7. You'll be looking for missed strobes, multiple strobes, or out-of-time strobes, at 1700 of course..