Rear bumper 67 GTO

I'm worried that the rear end of my 67 GTO is not right. I replaced the rear tailght section with a restopart. It fit, i thought, well. Except for the left side quarter panel trunk extension doesn't reach to the frame. This might make the rear bumper not be long enough to slide in around the quarter panels. Does anyone know the inside width of the rear bumper for a 1967 GTO?

Thanks,
LET