Since 43 years have passed I will speculate on the reason for the unusual decals. Wayne Andrews (another source says Max Berrier) drove a #15 highly modified Mustand in the East Grand National NASCAR series. As you can see in the picture the decals are pretty much dead on to the car as it was raced except for the side stripes which I suspect may have been added to the real car or someone at MPC just bought they would look cool. There are decals for different race tracks this car raced at on the sheet. Other pictures show a chrome front bumper and a 71/72 style grille and the car was raced in 72 and/or 73. So, either these decals were meant for an earlier issue of the kit (the version with the roll cage and race interior would make sense) and instead of bothering to design a new decal sheet MPC just threw this one in the new 73 kit. So in someone's infinite wisdom they decided let's make the same mistake and include a decal sheet that makes no sense for this stock 73 sportsroof. I would find some 71/72 grille parts (Detroit Resin Group has them) and use the decals on the Warren Tope kit and buy some Keith Marks 73 Mach1 decals and make a separate stock Mach1. I have no insider knowledge of my assumptions but given the bizarre things we have all seen with other kits and decals I'd say I'm not too far off. I will not speculate what engine Andrews/Berrier ran but a Boss 302 would not surprise me at all. Of course we all know that engine did not come in any 71-73 Mustang which is okay because the kit has neither the 351C or 429CJ either much less a pirated Boss 302. What you get is a regular 302 (single exhaust would be factory correct) and a Boss 429 which never came in any 71-73 Mustang.
Sort of a blend between a diecast and a model. The body is prepainted and decorated. The interior and chassis and engine are like a typical model. The nice thing is it was a very fast build. The 68 ten spoke wheels are incorrect for a 67 and the body is stock Shelby instead of having the modified Eleanor body kit. Nonetheless less it was a fun build and looks cool.