Worth doing! Tweak your suspension a bit, since you'd be putting a 119" wheelbase frame under a 117" wheelbase body. Biggest visual difference is the gas tank area. 65 has conventional wide, flat tank and 66 has spare tire well and conformal tank to match. Just cut and pasted that area of the floor. It would be a huge detail improvement underneath!
Jim, I now return your thread to its regular schedule! I am looking forward to this build; these are some of the crown jewels of GM as far as I'm concerned!