I bought an adjustable vacuum canister and a recurve kit. (Weights and springs) The vacuum canister was adjustable.....it provided between 29 and 34 degrees of advance......Way too much. The weights in the recurve kit looked pretty cheezy too. Maybe I bought the wrong brand. I just cleaned up the existing weights, used new springs, and got my $$ back on the adjustable canister.
I did quite a bit of reading on the subject and found that a SBC likes to have 36 degrees initial and 52 degrees total timing (if the cam doesn't have a lot of overlap)
End result: 16 deg base timing, 20 degrees mechanical starting around 1000 rpm, all in by 2800, and 15 degrees vacuum advance.
Doing this took me pretty much an entire weekend of fiddling around with the distributor, test driving, listening for detonation, fiddling around with the distributor some more, test driving……..over and over and over again.
So far, this has yielded the best results for me. I have about 50 degrees of total advance at 2200 rpm (my 60mph)
This tired old 114000 mile car runs better than new now, and the mileage has gone from a city/highway combined of 12.5mpg to 15city/20 highway.
I've had some naysayers say that timing adjustments could never make this much difference, but they CAN. I would even bet that I could tweak on it a little more and get better results, but for now, I think I'll leave it alone.