Ok, gp. 4.352 pn. 591436 that spacer acts as a bunper or end stop for the needles and if your haveing trouble with the assembly not going together it may be that it's those four long needle bearings that you said you had. So here goes one more time, if you don't have 18 needle bearings that are the same size, meaning diameter and length then somewhere down the line they got mixed up with another transmission and you're gonna have problems assembling it.
Take a look at the last three pictures in this album, they should answer your questions:
http://www.pbase.com/dennygraham/sm420_rebuild&page=all dg
