Timing

Fluctuating vacuum usually means leaking valves, not vacuum leaks. Combined with the fact that you hear air coming out of the tailpipes I'd say you at least have leaks in the exhaust valves. The % of leakage your getting from the L/D tester indicate you have real troubles. I'd be prepared to tear the whole thing down. 45% is waaay too much. Compression tests pretty much indicate the same thing. Although I trust leakdown tests a lot more since compression test only show peak pressures.
Good luck!