Depends on your mechanical ability as to whether to tear it down yourself, and a sense of adventure. I dont have a ton of experience with transmissions but jumped in and did mine myself. Works fine ! Be sure to get a service manual for guidance.
I tore down my overdrive 3spd and it needed several gears replaced in the 3 spd portion. I had another plain jane 3 spd and used it to get the pieces needed to fix the overdrive gear box. The big thing is to get a rebuild kit with all the bearings etc in it before you start. I sourced mine through Northwest transmission (located in Ohio). they had everything needed to do it, except a few other bearings like the front ball bearing, and the rear which I got through one of the big auto parts dealers, like Napa etc.