65 barracuda driveshaft length / transmission question

I can't help any with your driveshaft length question, but wanted to ask what engine you're putting in? If you have the 65 model 273, the register on the front of your 78 model 904 torque converter won't fit into the back of the crankshaft in the 273. The later cranks had bigger holes in the back of them.
The 904 you plan to use will bolt into the car just fine. You can use an aftermarket floor shifter, or find an automatic on the column, steering column from a 66 Valiant or Dart. Bonus if it's a V8 parts car with all of the linkage!