Yes, those are the best sizes.
The "M" number is the volume displaced by each rotation in cubic inches, with a pulley ratio of 2:1 to 4:1 to get the total near the engine displacement.
I have no idea of the nose length from the end of the housing to the pulley centerline, which I suspect may be different between manifold types even among the 3800.
My guess: GM-based 3800 V6 Eatons probably have good interchange of noses between specific applications, which is easy service.
I wouldn't be worried about 1/2" difference, but 2-3" or more might need some work.
I've heard (but no details) that the nose itself is not hard to shorten (both housing and shaft), and I'll bet there is a nice market out there for modified noses (as an exchange) to fit other non-OEM (fabricated) manifolds, like for the other obvious engines: slant 6, Ford 200, Ford 300.
