Old car HP ratings were very often overstated and of course were measured at the flywheel with no accessories, making them bogus in most cases, although some of the muscle cars fudged the 'real' HP numbers for lower ones so the insurance nazis and the feds wouldn't get antsy; NASCAR rules at the time being a factor, too.
To compare the car in question (Chrysler 383) with yours, subtract at least 30% from whatever the factory HP rating was (say, 335) and you get around 225 HP in a car that weighs almost 4000 pounds and has an automatic, not to mention joke tires and suspension and probably a 2.73 rear ratio (for gas mileage purposes). Are you intimidated yet? Didn't think so.
Your Uncle labors under the delusion that those old behemoths with the huge cubes and big HP ratings were actually fast. Nope.
They could get out of their own way and were great at higher cruising speeds but stoplight warriors they were not. Some of them were quick, such as the Pontiac Bonnevilles and Catalinas with 389's and the rather rare 421.
We all know about the 426 hemi Dodge/Plymouth and the famous Ford 427, as well as the Chevy 454 and so on. These were 'special' vehicles that were very rare, even back then, as they were expensive and rather impractical for daily driving. The run-of-the-mill Chrysler 383 was never a serious contender, even back in '65. Nothing has changed in 35 years.
Your uncle is mistaken here but so what?
If he won't race it's a moot point anyway but leave the challenge open and be careful of running him at some high speeds where the cubes and high rear axle ratio might give you a scare; that is, if the old boat didn't blow up. Some things just don't age well and cars that don't run for ten years are one of them.