Jeff, I believe the spacer you are referring to exists in the later (close ratio) FS5W71B that was available in ~1981. The synchro design was changed, and the three-pronged spacer (32624 - I think it’s actually called an insert retainer) was part of that and it is indeed the first thing on the mainshaft behind the mainshaft bearing - on the later model transmission.
Regarding the RockAuto kit, it does not contain any retaining rings. I assume you are referring to the "main drive bearing snap ring"? I just reused mine. I hope you can find one - it seems critical to the proper axial positioning of the input shaft relative to the mainshaft. When the mainshaft, input shaft, and counter gear assemblies are all fit together and pressed into the bearings in the adapter plate, there is some fore/aft movement possible between the mainshaft and the input shaft. When I was testing the rotation/movement of the various parts, I noticed that if I pushed the input shaft firmly back against the mainshaft, the whole thing sort of "froze", but if I allowed the input shaft assembly to float forward, everything moved freely. Once everything is reinstalled in the case and the front cover is attached, I think the big snap ring keeps the right fit between the input shaft assembly and the mainshaft assembly (which is locked into place in the adapter plate by the mainshaft nut).
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