welll, thats a d model so i dont know about the packards MP, but i know that for the allisons in the P-40s and early p-51s, Allison sent out a memorandum to the Brits flying the 40s in africa that they would approve 67 inches of mp since tests showed the engine could handle it.. that merlin was a big packard engine, so my guess is that if the p-40 could handle it, then, if for no other reason than to be better, the merlin could handle it.. I may be incorrect of course..