What is causing it to buck when hooked up to manifold vacuum is that you have it hooked up to manifold vacuum. Vacuum advanced are designed to operate on ported vacuum. You have a pretty good advance curve in it for a street driven big block. When you plug in manifold vacuum, it adds even more advance and screws up your advance curve. What you are doing when you plug in manifold vacuum is giving the engine full vacuum and centrifugal advance at cruise and no vacuum advance upon acceleration when the engine needs it.
FWIW, the last 2 BBC's I tuned lately responded positively to 39-42* timing for best HP and TQ, I think he may be light of the curve, but the 4 spd/3.08 overall gearing may be lugging and making things worse. I've driven undergeared O/A cars and they act the same.
The way I see "best timing" is it has more to do with chamber size/shape, stroke, intake tract design and trapped charge, some combo's just need more, the poorer the combo, the more timing generaly speaking.
And 90% of the time, warmed over or mild/well built performance engines respond better to manifold vacuum than grandpa's early 70's emision minded ported location. Specially when over cammed and under the required comp ratio.
@kdsracing, The last sentence, it would depend what "cruise" is, if it's under the centrifugals max rpm activation, you are not automatically getting full centrifugal, one is vacuum/throttle position related, the other is rpm, they operate on different platforms.
When you accelerate heavily and/or to WOT, you do not want vacuum advance, just the opposite, you want initial/centrifugal only.