Did you make a text file called "p40c1_b_d"?
Info inside should read like this. (this is the exact file info from my P-40E - feel free to try it on the C)
#texture name, Bump width, Bump height, U scale, V scale
# bump width and bump height are optional but must be present
# if U scale and V scale are used. U and V scale are also optional
p40e1_b,40.000000,0.007500
Red section is what's important. the first set of numbers is the implied length of the fuselage on the bitmap (IE the length of the fuselage in this case = 40ft) When I adjust this spec, I use changes in 5s and 10s - hense, why it's 40ft and not closer to the true length of roughly 30ft.
anyway, a better description is in this thread
http://bbs.hitechcreations.com/smf/index.php/topic,333609.0.htmljust experiment with the numbers until you get the effect you want.
Also, you may want to try actually adjusting the bump map itself if you find some effects unbalanced (IE wanting subtle rivet detail but a prominent rudder trim actuator and finding them actually the same in game)
Hope that helps a bit.