904 to 727 upgrade.

You need a shorter driveshaft and the larger yoke for the 727. The shift shaft and kickdown is all in the same location so that can stay the same. Trans mount is the same.
Not cheap but I bought my last shaft from dennys driveshaft. They have a form you fill out with all the measurments they ask for and they build it. Dennys Driveshaft and Driveline Parts High Speed High RPM Balanced Steel and Aluminum Drive Shaft Specialist with Free Shipping on parts orders over $99.00
Check out mancini racing, they also have driveshafts at various price points. Another option, since you need a shorter shaft is to have a driveline shop shorten yours.
904 vs 727 debate. Yes there are plus and minus for each so it really depends on what your doing with it. Race guys like 904 because of less rotating mass to eat horsepower. For durability you cant argue with the extra clutch surface of the 727. I like 727's but I wont build one without a bolt in sprag since I had a sprag failure. I lucked out but from what I hear it is rare for a 904 to grenade the front drum with a sprag failure. Its quite common on the 727 and it aint pretty. So there's that argument too.
For our lightweight A bodys the 904 should be more than sufficient.