The front band is solely responsible for 2nd gear application so the issue is with it or it's apply servo or the accumulator. Here's my list of possibilities.
1. The valve body to case torque is 100 inch lbs. Not 58. It may not be sealing good allowing leakage to the accumulator or front band servo.
2. The sealing rings on the front band servo may be leaking.
3. One other possibility is the front band is glazed so it doesn't hold good. It would be really rare but a possibility.
4. If it has a real soft spring under the accumulator that'll make the 1-2 shift slow/soft.
Why it seems to shift good when you manual shift it is because when you do that the rear band stays engaged longer giving the front band more time to apply. Normally on a stock torqueflite if you manually shift it they'll experience shift overlap.
BTW: 1 turn on the pressure regulator isn't much in terms of a pressure increase. I highly doubt it's a problem.
