Hi RivinitUp, I did not modify the 65 bumper in any way, I did fab my own lower quarter pieces that line up with the ends of the bumper which turned in a little tighter at the bottom of the quarter panel.
Hi RivinitUp here are a couple pics of my 63 with a 65 rear bumper. I trimmed out the old taillight panel and fabbed a new one out of 16 gauge sheet and also fabbed a shelf for the 65 taillights to clear.The 65 bumper brackets line up just fine.