Riddle me this..mechanical vs vacuum advance

There is no 'riddle' about vac adv v mech advance. They do different things, work differently, but both change the ign timing. Bit like a cam & headers: different things, but both can add power to the engine.
From the specs that you provided, this engine is going to like a lot of timing at idle. Maybe as much as 30+ degrees.
Chrysler rarely made mistakes, but they did by not using manifold sourced vac adv MVA ]. They used the useless ported vac adv. MVA does everything PVA