As with any swap, a V-8 powered parts car of the same age as yours would be the best starting point, so you could swap anything that's different. But these are getting a little harder to find (they're out there so keep your eyes open). Here's a basic rundown of what's needed.
K-member: The 63-66 cars all used the same k-member, so you're fine there.
Steering: The V-8 car center link had a deeper drop to clear the oil pan. Some say they've been able to use the 6 cylinder piece. Commonly 66 cars had the V-8 part, so check what you have.
Radiator: Your core support will accept a standard 22" Mopar radiator. These come in two basic varieties pre-1970 and post-1970. The difference is the location of the lower hose. You can use either one, but must match the water pump, pulleys and brackets. The early iron water pump will bolt to a later engine, but it's lower hose will make the timing marks a little harder to see (not impossible).
Exhaust: The factory 273 iron manifolds only support 250 HP or so, barely adequate for a 2 bbl 273. Headers are a bit of a PITA. Dougs and TTI sell underchassis long tube headers specific to these cars. They fit pretty good, are well made and cost accordingly ($$$$). Hedman makes fenderwell headers, best for a race car, but plenty run them on the street. You have to cut sheet metal, but the cost is substantially lower. There used to be some short style "Spitfire" headers. Made by a guy in his garage. Forget trying to buy these new. Some claim to have adapted various other manifolds; 340, Magnum, later A-body. Never seen any of these myself, so I can't speak to them.
Throttle linkage: The /6 rod type wont work. Look for the 64-66 V-8 cable and pedal bracket. I've seen later car pedals adapted, drill and cut...
Drivetrain: All these cars originally had 904 transmissions, and they fit best. A 727 fits OK but may require a little clearancing of the floor, nothing major. The 7 1/4" rear axle won't last long behind a 360.
Brakes: Most /6 early A-bodies had 9" drums. These were barely adequate when new, and parts are getting scarce for them now. You also have a single circuit master cylinder, which should be upgraded to a dual circuit system. Some will say you can get by with 46 year old, inadequate brakes "for a while", and perhaps you can. Just don't follow me in traffic. UPGRADE
Suspension: Again, there's a safety aspect to doubling the power and keeping 46 year old worn out parts. At a minimum, rebuild the front end and install .870 (commonly available) torsion bars. There are many upgrades from there, but first build a good foundation. The rear is probably pretty tired by now as well.
Plenty of info on this site,the "Search" button is your friend.
