mxpx, make sure you get the outer o-ring and the inner seal as well. There is also a small retaining clip that goes in the housing. You may have known all this but I hate buying something and then have to go back to the parts store for more stuff. Marc
The TH350 PN is 345215 superceded to 25513247 and it's different than the TH400.
The Chevy version is 7/8" diameter and the BOP version of the TH350 uses a larger one at 2.1" diameter.
The Chevy TH400 is 1.935" diameter and there is one for 34-39 teeth gears and one for 40-45 tooth gears.
I don't have the part number. You'll need to find a parts counter guy who knows how to find it.
Last time I did that search, it was called a "gear holder" in the computer. It comes in a Delco box and has the seals installed but may not have the outer O-ring. I think I got the O-ring at a local trans parts wholesaler and come to think of it they may also have the speedo gear "bullet" as some call it.
If you get the PN let us know and I'll post it in Best of tech. This one comes up about every six months or so.
Items in picture are from upper right going counter clockwise:
Driven gear holder, sleeve or bullet
The TH400 gear holder has the gear range cast into to housing (34-39 or 40-45) it also reportedly fits the NP208 transfer case.
update Part number is:
1362294 for the 34-39
1362293 for the 40-45