OK:
ModAct 5.3, 4.12.2

(as if that was not obvious=MA 5.3=4.12.2)
The P-51
D pits have the new meshes, but miss the mirror, and canopies won't open.
In DBW 1.71, 4.10.1m...
The P-51D-Early and -D-NT, 1944 show no cockpit at all, I get this error log:
(missing line -HUD
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[2:23:21] Missing Hud customization ini file (or missing lines in it)
[12.10.2014 2:23:25] -------------- END log session -------------
All other cockpits in DBW seemed to show the default pits.

To me that says that maybe indeed it is best if you extract your entire cockpit used and apply the files in that, to make sure there is no mix up.
When I tried to mix e.g. F_Red's old P-51 cockpits with other mods for the early Mustang's pits, similar issues,
I assume for pits there is a danger of incompability if Mesh files are used.

Make that double

as I'm not sure.
I assume or am quite sure that if there is a mesh file entry which loads before another pit of the same type with a conflicting mesh or hier.him entry, such things occour?
In any case all Corsair and P-51B/C cockpits worked in all installs, a noted imcompability would be Freddy's old P-51B/C pits as they use a different mesh setting, with combined meshes.
Hope that helped a bit, to sum up:
..if named properly, only the P-51Ds so far (in my installs of course) are problematic, rest ok.