RE: Hangar 9 150 Size P-51
Tom,
My LHS's carry reversing Y's, there are quite a few companies that make them but I never did a search for one on the web. I avoid using them if at all possible. A reversing Y actually has a little screw adjusting pot where the cable branches that is used for trimming the servos together.
I forgot about the trick about reversing the servo mount- this is also a very good option (better than using a reversing Y).
I'm looking forward to hearing how your flights go with the trainer!