Since you mention timing and vacuum advance, I recall having a similar issue when getting my truck on the road where it seemed to bog/lunge when cruising on the highway at a specific speed.
When building my truck, I installed a new GM HEI P/N 93440806, which came with an aggressive vacuum advance (added 20 degrees at 7.5" per a spec I found on-line). Mechanical advance for that HEI at 2400 RPM is rated at 16 degrees. After messing around and trying out different combinations, I figured I was getting too much total timing at high vacuum/cruising, so I changed the vacuum can to a Standard Motor VC204: 14 total degrees advance, 10-12 max advance, starts at 7-9". That really seemed to do the trick. I'm running my vac can off of the manifold port, and have 12 degrees of static timing (IIRC). I've tried it both ways and my engine prefers the canister on manifold vacuum.
Since then I've had to replace the GM coil and module and went with Davis Unified Ignition (DUI) parts, which really woke up the engine, maybe not related but it runs way better than it ever did on the stock GM HEI.
I know my specific details likely don't apply to your situation, but thought it might give you a few ideas. Maybe block off your vacuum advance and see if there's any difference, give it the ol' suck test or replace it with another?
Here's a good explanation & reference for some common GM vacuum canisters, their specs and typical applications:
http://outintheshop.com/faq/Vac%20Adv%20Spec.pdf