It sounds like you are using the 737-300.ac file that you had altered in AC3D.
Have you got a backup unaltered file you can use? If not download the file again and use that.
This is the method I use.
Copy the "737-300.ac" file and the "733UAii.rgb" texture file into a new folder and use that for working in.
Open the "737-300.ac" file in a text editor and change all instances "733UAii.rgb" to "733UAii.png" then save it.
Convert the "733UAii.rgb" or the blank template to an .png file and open in Inkscape, then use this as your base layer.
When testing the file, export it as "733UAii.png" and open the 737-300.ac file in 3dviewer, you keep doing this until you get it right.
When finished, convert the png to rgb (if you are using cvs there is no need to do this) and write the .xml file and go flying.
This way there is no need to touch the original .ac file.
I hope this makes sense to you