These planes have lots of torque to cause a left turn on takeoff. In general the P-51 (model or full scale) is known for this. Generally expect to hold lots of right rudder to counteract the torque on takeoff. The torque is less of a problem if some elevator and throttle management is used. Go easy on the throttle to get it rolling. Once it starts to roll don't hold any up since the high angle of attack created by holding up will increase the torque. Gently add some up elevator once it is at flying speed. You will still need a good amount of right rudder to keep it from turning.
What retract were you using?I'm confused why changing it would solve this type of problem.
Tried to maiden the P51 2 weekends ago and had an issue with the retract tail wheel on take off. plane kept wanting to swing around. Is there any torque that would pull this plane to the left on take off? I have a GP Christen Eagle 2 .46 and that thing pulls hard to the lefty and needs lot of right rudder.
But replacing the tail wheel retract with a robart air unit and having servo to activate tail wheel doors. ill take some pics once the install is finished.