This is actually word-for-word what my comment was gonna be!12 bolt would be the no brainer.
I had a '65 Impala convertible with a 327/250 horse and a 3 speed on the column, that car had a 12 bolt. I had it almost 20 years and never had an ounce of guff from that rear end.I put a 340 horse 409 and a 4 speed in it for a lot of that time.If your 65 came with a 300 horse 327 or larger engine then it would have come with a 12 bolt.Smaller than that,6 cyl.,283,250 horse 327,it would have a 10 bolt 8.2 rear.
If Chevy had used them in their big cars, they would have been the rear that the aftermarket flocked to because the numbers of them out there would equaled or exceeded the numbers of 9"s, I know they were more expensive but economy of scale would have offset some of that.Have a friend with an off topic GM product that finally broke his 8.5 10-bolt and replaced it with a bolt-in 9” and the car lost .20 at the dragstrip even with ratios being exactly the same. Today’s Fun Fact: that 9” low pinion centerline (in relation to the ring gear) is a horsepower sponge and was developed for FoMoCo products to have an unobtrusive driveshaft tunnel in the passenger compartment. The lower the pinion CL, the more tooth contact & obvious strength but there’s gonna be a HP price. In a perfect world, the early Olds/Pontiac should have been developed more by the aftermarket and that probably would have happened if it stayed in production cars longer.