On a stock stroke 340? that vacuum figure for that much duration seems unusually high to me. two possibilities come to mind. super wide lobe separation like you'd see on a blower or nitrous camshaft.
OR a solid lifter where about 10-12 degrees of the lobe duration is taken up by valve lash.
regardless, the point remains... everything else being equal. more duration and a tighter lobe separation will increase overlap and reduce idle vacuum. Your personal anecdotes are interesting, but there's just no disputing this fact as it's probably the most basic rule there is about camshaft theory.