what is needed -convert 65 Dart 273 to 340 or 360

Port match the exhaust manifolds, or use Dakota truck exhaust manifolds or 340 hp manifolds. The 273 driver side motor mount will work but the passenger side motor mount will need for you to weld a tab on it to put another bolt through it. You might also need some washers for spacers. Although the transmission will bolt up, if you are changing the drive shaft you might as well change to a 66 and later V8 transmission that had a slide-in yoke and does not have a bolt on yoke. I would also go with new rear leaf springs (shackles, bushings, bolts) and put stiffer front torsion bars on the car as well as better, bigger brakes to stop the more horsepower that you will be making. Don't forget a radiator that will both cool the engine and have the right Inlet and Outlets on it. Your old one may still work.
You can use the 273 oil pan and pickup tube on a 318 or a 340 to make it fit properly in the K frame, but a 360 car pan may require some minor massaging done to it to get it to fit, like some dents on the right front. Truck pans won't work at all.
You don't need a V-8 steering as you already have it and you don't NEED a notched trans crossmember for dual exhaust, but you can do it.