As far as I remember all four cylinder Medallion used the NG9 transaxle and all the V6 were automatic and used some type of ZF automatic transaxle; The NG9 was also used on the Renault Fuego and it is too small for any V8 application.
Here is a picture of both side by side, the UN1 is on the left and the NG9 on the right, you can clearly see the difference between the two, The UN1 came from a 1995 Lotus Esprit S4 (300 HP I believe) and the NG9 is out of a Renault Fuego, about 100 HP on the Fuego?
In any case the only difference between the Medallion and the Fuego transaxles was the bell housing otherwise they were the same, they may had different designations, NG6, NG3, etc but all the NG are too small, translation, NG = no good, you will need a 369 or UN1 transaxle.
The only two production cars sold in the US that had these types of transaxles were the Lotus Esprit (UN1) and the Delorean (369). The Delorean was a rear engine car so if you find one you will need to open the box and flip the ring gear. The UN1 on the picture will be going on Ebay soon but to be honest with you a better or perhaps cheaper solution here in North America Is finding a good Audi transaxle. The UN1 is a good choice for Europe or for any GTD owner, as these cars were designed around UN1 and 369 boxes.
Hope this helps
Pedro