You are correct about the grill. It is completely wrong.
All Johan did was alter the 74 and 75 front grills for the 73 body.The rest of the body is nice except for the light details for the back panel and marker lights. The seat pattern is all together wrong. It's not correct for 73 either. 73-74 promos are still available on ebay all the time. Johan made a limited run of decals for the dash and light details. The 73,74 and 75 grills are all incorrect for a 442 or Hurst/Olds. 73 used the same 442 emblems as 69-72. The 74-75 used differrent ones that are 74-75 only. So a 442/Hurst/Olds hood along with a detailed dash and correct grills would be nice. New decal sets would be nice also.
I've found the AMT/MPC chassis works the best for these but thats just me.