Yep, if the FT Mustang is really 40" span that'd be 1/11.1 scale.
Scale is just the full size in inches divided by the scale size. 37'x12 gets you a full scale span of 444" Divide this by the 40" span of the model and you have 11.1 So the scale is 1/11.1 scale.
Well you can do what I've done in the past. Use Photoshop or GIMP if you need a graphics program for free. Draw up the decal and Print it on the clear inkjet material. Make sure you print it inverse then cut it out like a decal. Once you've done this spray some spray adhesive on the printed side and then stick them where you want Viola instant professional looking sticker or decal.