That is very interesting they must have forgotten to paint that one before the engine was put in and when back or something lol. Mine was not like that but I dont remember the wheel wells being body color.
I think it was more about the fact to save paint, the painters were instructed to only paint the tops of the engine compartment, or what you could see after the engine was installed.
I have 5 '75 cars, and they are all like this.
