I have a universal brushed DC motor
What motor? is there some data about it? Universal motors for mains use are not necessarily OK on DC asthe inductance of the field windings limits the current at mains frequencies - so some further testing to ensure it isn't overheating on DC would be wise.You'll probably want to disconnect the field windings from the armature circuit and provide constantvoltage to the field and only PWM the armature, should keep the torque reasonable at lower speeds.
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.
This is a "universal" motor and you should be using a "router speed control" with it. The dark blue and light blue wires connect to the brushes, the red wires appear to be a tach output, and that leaves the brown, orange, and black wires for, likely, two ranges of speeds. You could guess that the black connects to the AC neutral line and the brown / orange allow to ranges of speeds, but without documentation it would be dangerous for anyone to make a straight up recommendation.
Below are some motor controllers that might be of interest.http://store.qkits.com/category.cfm/DCMOTOR