You have a non-synchromesh first gear in that '66 Ford. When going for First or Reverse, put it in Second when you press the clutch. This stops the shafts from turning, then, without letting up the clutch, pull 'er to Low. This becomes instinctive after a while.
I would suggest a T5OD standard trans, or a 4-speed OD from a Torino or a newer half-ton. You'll never get the mileage from an automatic like you will from a stick, and I'm not just talking gas mileage. I have a 62 Unibody with a 351W and four over with 3.78 gears. Works great.