Geomar missed big time since the automatic trans overdrive system and various remedies he described so well does not apply to your manual tranny system.

Since you will be removing the OD to replace it, there is no need to actually dismantle the OD any further than removal from the tranny. While the OD is off, replace the two O-rings on the actuating pistons. Fluid pushing past these is the most common cause of slipping overdrive. Second is cuased by a condition inside the solenoid which is cleaned by the process below.

Then, to ensure there is no other problems, remove the solenoid, dis-assemble it by removing the innards via the end of the brass barrel. A circlip allows removal of a plug, spring and piston with two small O-rings from inside the barrel. An iron slug can be pushed up and out of the way with the extension nozzle on a can of spray electrical contact cleaner. Leave the nozzle in to allow complete draining of the fluid and allow to air dry. Minimal lube to re-assemble.

IF you find physical damage which requires parts replacement, then you can be able to remove the speedo drive gear and rear case from your 79 and transfer them to the 84 unit. You would know this in advance since the fluid would look grey or have streaks of grey in it from the grinding to dust of the cone clutch.

Use this link as a start for the OD dismantle.

http://www.brickboard.com/FAQ/700-900/OverdriveRebuild.htm