My two cents and personal experience-
I would not run the 6X heads. I believe they are smog heads they have a 2.11 int. with a 1.66 ex. and larger CC.
Find a set of RAIII or the RA II series - #48, 16, 12 or like heads. They feature 2.11 int. valve and 1.77 ex valve. and 72 cc’s. Don't do anything to the combustion chambers. These heads are still plentiful on the internet/ wrecking yards or just laying around in Pontiac guy’s garage. Port match/ clean up the ports. Go to Wallace Racing website and look at the head codes. All head specifications are listed…..do your homework before you buy/ build.
Run a good flat top forged piston with any of these heads.
Cam- Crower solid #60311 or the hyd #60211 (I personally ran the #60311).
Edelbrock torker or victor jr. intake. Run a 750 -800cfm dp sq bore Holley
Hooker 1 ¾” primary tube w/ 3” collector with 3” full exhaust.
3.73 gear with a “good set of street tires”.
If you have a 4 spd, your 65’ will run/ should run bottom 12’s or crack high 11’s on a good set of street tires……. I know, I ran the combination years ago in my 64 GTO and the car was VERY streetable and ran the numbers.