headers that don't fit are DEFINITELY a problem to be dealt with, a while ago, the Hot Rod shop I worked at brought in a '69 Mustang Mach 1 with a Coyote engine conversion where the headers wouldn't fit, all the new motor mount welds looked like bird sh*t and the "fabricator" who did the conversion had no idea of the concepts of "center line" and "square", it cost the car's owner tens of THOUSANDS of dollars to clean up this mess.