You can use ball links and adapt them on the retract itself. They connect the doors directly to the retract just like in Chads post. The P-51 the Corsair and other planes share the same concept with linkages connected to the retract and the door. With your length mod you should still be able to do this. It just comes down to timing. Where the linkage is placed on the door relative to the retract. The pivot locations and linkage length will need to be experimented with until you find the right length and position.
Ive done this on both TF P-51 and Corsair in the same way plus I lengthened the Corsair retract too.
The full scale Corsair tail gear doors pivot out about 180 degrees so keep that in mind.