The Tstripes on the AMX (Javelin) is what I put on my SST of the same year. I had mine painted on and kind of had the same problem as far as dimenisions were concerned. What I did was to find a print ad from AMC that showed the wrap around stripe. AMC originally was going to do the wrap around but changed it at the last minute on the production cars, but by the time they had changed it, the print ads were already out. ANyway, I took a copy of the ad to the painter and he shot the paint on using the ad as a stencil. Seem to work out very well.
There is a small stripe off to the side of the main stripe (about 1/8 inch wide) that noticed in the print ad and that was put on with using a pin striping tape. Came very well and havent had any problems with the painted version of the stripe itself.