There are a few 'things' that will affect the vacuum. First of all, are the end carb throttle plates closing completely? While you purchased new or rebuilt bases from a, hopefully, reputable shop, that still doesn't mean that those butterflies are set right. Then there is initial timing - is it correct. can it be jacked up a degree or 2 without ping? Also, are you sure that the harmonic balancer is telling you where TDC really is? That's a 57 year old engine, and those balancers are seldom considered in an overhaul. Next would be the camshaft - what is it? Stock? Mild? Beyond mild? Next would be the actual valves themselves - while they may not be 'burned to crisp', the seats, unless someone has changed them are not hardened for unleaded or even very low lead gas and may be starting to erode. I have to assume (dangerous) that you have checked those very fey vacuum lines, carb base gaskets and air horn gaskets.

Now, with all that said, 15 in Hg, if steady, no blip, slow drop, and with a triple carb set up isn't really bad - though I would be expecting 17-18 if everything is ok