Do you have bearings or bushings. I think all GD longtails have a bushing at the end of the shaft. Motors build before 2001 have a bushing at the motor end also. Most of the information I have seen indicates that the upper bushing or bearing hardly ever needs to be changed, they just don't carry much load. Just change the upper seal. The lower bushing needs to be changed if you can feel any movement of the shaft in the housing. Is there any vibration or is the unit using more than 10 grease gun pumps after a 2 hour run.
Seals, bushings, bearings, will only cost around $30 to $50. IF you are replacing the motor I would go ahead and replace all of these parts.
If you are getting a new prop, I would also get a new shaft. It runs around $80.
Here is a link to the service manual that will give the information you need.
http://www.godevil.com/documents/Go-Dev ... al_000.pdf