Would this 727 fit in my 1973 Duster?

There were 2 styles of torqueflites used in motorhomes. The standard car type and the short tailshaft version that usually has a yoke bolted to the output shaft. The one in that add is the short shaft version only used in motorhomes and HD trucks. You might be able to make it work but it's not going to be a direct bolt-in by any means. A custom driveshaft will be needed as well as some sort of adapter to make it bolt to the crossmember. I'd find a car or 1/2 ton truck torqueflite and save a big headache.