When you have any type of sizeable cam with more overlap they need more initial timing to idle well and to get your carb. idle set up right, but you can only go so far with the initial, (mechanical advance), or your starter will hate you. So you put your vacuum can to manifold so you get the initial that you need for idle without hard starting. In turn when you are now started and running you now can turn your idle down to get your t-slots set right, (holley carb.) so they are not pulling fuel at an idle, (.020 to .040 open). Then you curve for a fast advance, (all in by 3,000 rpm or so) and that takes up for the loss of vacuum advance that drops out, (from vacuum loss), the minute you floor it from idle. Then you check your total advance, (32 to 34 vortec heads), (36 or so non vortec heads). Then you limit the total advance, ( in the distributor) to hit those number's and you are good to go. Then the vacuum comes back for a good cruise as well. Either way (give the engine what it wants) so you have to try all kinds of different things when it come's to performance.
On a side note, when I was young and trying to learn about engine's I had a few stocker's that was trying to make fast, ( with no little or no money), so I would pick off used performance parts etc... I had a 71 impala 2 door and put a eddie intake, header's, dual exhaust and a holley carb. on it and it pulled pretty good for the day and age. Took it to my engine guy and said, ( what can I do for more, cheap). He then showed me his trick.
Full manifold for the vacuum can, ( vacuum cans have different advance amounts and vacuum activation point's). Give the engine what it wants
Speed up the timing curve, ( all in by 2,800 to 3,000 rpm). Give the engine what it wants
Limit the total, ( 30 to 36 ). Give the engine what it wants.
So this is what I did, (he really did this and showed me how) and my stock inside impala with some used bolt on's pulled harder off of idle and stabbing it from a cruise as well.
I did this same set up on a few stocker chevy's threw my younger year's with the same gains. My dad bought a brand new 1978 k10, (when I was kid) and took it to our engine guy for the trick timing with full manifold vacuum set up with only 800 miles on it and he told me it pulled a bit better.
Latter on in life most of my drag car's I just locked the advance at the total I wanted and that was it but not for a street car.
If you have any type of stout cammed street car or truck you petty much have to need to run full manifold vacuum to get your idle correct, (I have to with my k5). I would do it for stocker's as well but thats just me.
My dad went to high school with our engine guy. He was a gm dealership mechanic his whole life and when he retired from gm he opened his own shop and built engine's for the outlaw sprint car circuit guy's. So I was at his shop trying to learn **** all the time.
I am not bashing ported vs. full vacuum, just throwing out what I have done. Back to , (give the engine what it wants), and every opinion is different so you have to try it for yourself, for me it's full vacuum when set up right.