That power supply will produce an output as rough as a badgers bum. I also doubt very much if it could deliver 35A for any length of time (10 amps might be nearer the mark)
As to why PWM is better than variable voltage. Firstly for a DC operated field excited motor you need a fixed field voltage to maximise torque output. The ask yourself how you intend to vary the armature voltage. Assuming you want to produce an output of say 5 volts at the full rated current of 35A you'd have to "lose" 19 volts across the regulator. That means a heat dissipation of some 700watts. By using PWM you can get down to virtually zero drive at full current with virtually no heat loss.
You stated that the field in parallel gives the slower speed and more torque. You have learned one of the fundamentals of DC motors. May I suggest you read up on DC motors and their control as this will then guide you towards achieving what you want.