Can't help you with the leakage problem, but the dry black flakes out of the tail pipe on startup are probably an indication of a very rich fuel/air mixture or perhaps some soot/carbon deposits being blown out that may have collected as a result of oil leaking past the rings or valve guide seals. When the carbon first forms it may adhere to the inside of the hot header pipe. When the engine is shut off, the carbon may cool more rapidly than the iron header, causing it to break loose partially. Next time you start it, the hot exhaust gases dislodge the flakes and blow them out the back. (just a guess :p )
How big are these flakes?
Does it only happen on cold starts?
How many miles on the engine?
Has your gas mileage gone down or your oil consumption gone up lately?
Maybe someone with more knowledge of this engine will chime in with a better idea.