[jeffko]

I would rather say you are trying to do something difficult and really should be performed on the dyno. Most of your assumptions are correct and what you are trying to do is a great theory and the best of both worlds. Unless you have a 2000 lb trailer to haul around or a very long steep hill (which would involve lots of trial/error and some guesswork) you will not effectively get what you want without a dyno. Exhaust sampling (and I don't mean air/fuel ratios) and power can be measured through the rpms with the car sitting still and the ability to adjust. You simply can't compete with a dyno. Anyway here are a few things to know:

Use ported vacuum because it is indeed special. Direct manifold vacuum will add timing to your initial timing at the damper at idle and nobody with any idea about what they were talking about would suggest that. The ported vacuum will give you the throttle position which indicates low to mid throttle load. This is a huge distinction because it advances the timing as you use more throttle.

Initial timing is best to start around 12-16 but your engine will have a preference which is most easily indicated by a vacuum gauge. This is also the time to set your carbs idles screws again watching the vacuum gauge.

You should have all your timing in by 3000 and no earlier with also using vacuum advance. I would not exceed 38 total.

By the way, you will feel a better off idle to half throttle response with vacuum advance since that is exactly where it operates. Mechanical advance is simply that and is dependent on engine speed only. So your theory is correct. Using ported vacuum your vacuum advance will react more quickly and fall off with the loss of vacuum at wot which is where the mechanical advance will have taken over. I have to tell you though, you will find it very hard to find a dyno guy that will want to tune this way and has the co, carbons and nox testing for the efficiency of your combustion by sampling the exhaust. Dyno, air/fuel and combustion sampling = power, economy and peak performace. But until you find this guy (looking myself) you will pretty much be chasing your tail. Good luck and keep us posted.