I need to get a new rear drum for my '67 small block 727 and am finding decent deals on entire drum/planetary assemblies from other transmissions. I still have a 3 pinion planetary so seems like a good time to upgrade.

Would I be safe to drop in an entire 4 pinion planetary + ring gear + drum from a 904 like this? https://www.ebay.com/itm/304575839320

The only difference I can see visually is that this doesn't have the thin splined washer that was in between the planetary and the ring gear on my original.

I have read that there was a change in the spline taper sometime around 1976, but I can't figure out if that would affect what I'm replacing or not.

No need for 4 pin planets. The weak link in the planet assemblys are the splines in the aluminum front cattier. With big enough power they will eventually strip where they fit on the output shaft. I've never seen a plantertary pinion break. Only a very few planetary needle bearing failures. There are early and late rear drums. Some use the thin thrust washer and some don't. Make sure the center thickness matches. If only that band surface is poor you can trim them in a lathe. Many times crocus cloth will clean them up..
Doug

Hey Doug, I think you meant to say the rear annulus - early and late. The rear drum never had a thrust washer. It semi-floats. Can’t go rearward because of the ORC race and can only float forward a bit and gets stopped by the rear planetary tangs and a step in the drum.

You are correct about the annulus. Last year I had a rear drum that was different. Don't know what it was out of. But it didn't fit. It had a thicker cross section in the center. it bound up the rear assy when installed.
Doug