Changing a 3 on the tree 1964 dart to an automatic

That’s a pretty easy swap - I would use a 66/67 904 with the matching small snout torque converter which will fit the recess in your crank. You didn’t say but you’re probably looking for a transmission to fit a slant. Add an aftermarket floor shifter. You can even continue using your current steering column (you’ve already removed the linkage at the bottom of the column). Remove the shifter handle if desired. You can even use your original brake pedal - push the clutch pedal to the floor - the spring will keep it there.

EDIT- no welding involved!