Basically, the Trumpeter kit of the C47, which I must stress I've only seen photos and reviews of, is quite inaccurate in the shape of the fin, rudder, and extreme tail area. There is even an aftermarket replacement for this. Personally, at four times the price of the Revell/Monogram kit, I don't think it's worth the money, however detailed the interior is (not much can be seen anyway in the cockpit, and the main hold wasn't exactly chock-full of detail on the real thing), or however 'nice' the exterior surface detail - this will be mainly hidden by paint anyway. I think I'd much prefer two or three of the R/M kit, which looks good enough to me, than one Trumpeter.
Anyway, on to the P61. If you men the extreme front of the cowling, where it surronds the engine itself, then this is mainly a streamline fairting, although it would also enclose theexhaust collector 'ring'. I haven't actually seen this kit close -up, so I'm not that sure what's what, but I would think it's easier having one engine bay open, as this would often be normal if only one engine needed attention, or the other engine had been 'finished' and closed up. If you want to have both cowlings closed, then the kit's cowling panels should be designed to be made to fit in the closed position also, although they may need a little filler to give a more realistic appearance when closed.
As for aftermarket detail parts, I wouldn't know without searching the net for them, but both 'Maglar' and' Night Fighter Nut' have recently built this kit, so they might be able to advise you.