Yea, you probably are best to replace it, a good used one isn't very costly. Coup and Sudan 280SE have the same transmission in it, unless special ordered. The 4.08 is an American 280SE rear end.
I'll get you the Transmissions # when I get back in town. If it were me, If I had a good 280 S transmission to use, I would do it!
Think about the rear axle change, I spent $300 dollars on one, and a friend and I put it in, took 3-4 hours. There is so much to benefit by doing it. Far less wear and tear on the engine, better mpg, much more quite running, even a little cooler running.
Negatives are, incorrect speedometer, and less bite at the low end.
I'll get you some Trans #'s later in the week.