Here is the schematic for my I-5 4QD150-24/36 controller conversion. This model controller is four-quadrant to provide dynamic braking and doesn't have any of the relays which some other members have found troublesome on other 4QD products. It is jumpered for "wig-wag" mode so that the throttle lever can be used to switch between forward and reverse (center stopped) using a modified E15 throttle assembly connected to a 90 degree potentiometer. The controller has acceleration ramp control so there aren't any issues when switching directions (very similar feel to a hydrostatic drive).
I run the motor full field and have found it to be more than fast enough for my needs. It wouldn't be difficult to add a field resistor and shorting switch for a high speed mode if needed.
I have some pictures of the conversion but now realize I didn't take nearly enough photos.