Evidence of mistakes past

[vette-kid]

I've mentioned before that I believe the car was hit at some point.   Here is the worst example of that,  passenger frame horn is about a half inch further out than the driver side.   I haven't measured the frame yet, but I suspect it may actually be more of an issue with the fenders being mounted poorly than the frame being bent.  The drivers side fender shows signs of a weld repair at the seam and the license plate bracket was mangled pretty good.   

 

As it sits, the passenger side frame horn will not be completely concealed by the cover or rubber pad.   I'm trying to determine a good measurement that I can get to to see if it's in tolerance or not.   Has anyone done any frame measurements on an assembled car?  What are the easiest points to access to determine square?