I agree. These cars are forty years old. If the transmission is out for any reason, this is the time to replace all the seals. The same is true for rear axle seals and other seals as you go through the car. If you're going through rear brake shoes, take a bit of extra time and do the rear axel seals and put in new brake cylinders.

The rear main seal and transmission seal should be replaced when the car is that far apart because they are so cheap compared with the labor to visit the issue later. I think the same is true for transmission seals for the clutch packs. TH400 or Th350 seal service is simple work once it's on the workbench in front of you.