What's wrong with the servos that came with the DX-7? Or did you buy just the Tx and Rx?

I'm not familiar with any model #'s except Futaba. 3001's would be a good choice for the H-9 P-51. They're relatively inexpensive, are ball bearing, and have enough power (torque) for your needs. Or the equivalent JR servo. Both are compatible with the DX-7 system, although you have to trim the little tab off the Futaba connector.