65 Dart 273 to 340 ?

You will need to weld a triangular shaped piece to your right motor mount Then drill it through so the new tab allows you to catch three of the ears on the 340 block. A few washers will take up the slop in the left side mount. If you use the pulleys and such from the 273 it makes the swap easy. The cast water pump puts the radiator hose in the proper place. The 904 will handle the 340 (if driven within reason) but you should consider a fresh build with a Transgo TF-2 kit and maybe Alto red clutches and Kolene steels. The hub diameter for the torque converter is different too. They went to 1.81" in 1968. You can get a bushing to adapt your torque converter from TorqueFlitePatty on E-bay or put a 68 or newer input shaft in your tranny and use a 68 or later converter.The main issue you will confront will be the exhaust. Headers (TTI or Dougs are expensive) (Spitfire shortys can be tough to get) are one solution. Keep the 273 manifolds. (Chokes up a 340) Or figure out an alternate route......
The ball and trunnion front U-joint has held up for me in various situations. (After all they held the Hemis in heavier cars)
If you have more questions PM me. I did more than a few 340 into 64-65 Barracuda swaps back in the day......