Repro wheel well and carriage bolt fitment

[stangtjk]

As I was taking apart my blazer I had a hard time removing the wheel wells because the carriage bolts would just spin... The wheel wells were new installed by the previous owner and he used 5/16x3/4 carriage bolts which I believe are correct and the square holes in the wheel wells don't appear to be wallowed out. So I was wondering if the carriage bolts should have been 3/8" or if the holes in the repro wheel wells are just oversized?