Do you know where he put 5th gear? The way I see it is you can only have a 1:1 gear is using 4th. No need to go through the cluster to do that that I can think of.
As I posted earlier, the only thing I know of that allows the gear box to not stay in gear is the struts and strut springs. Sometimes a syncro can do that, but it usually grinds a bit, especially when downshifting.
That little strut cams over center when the slider moves to engage the gear, and should stay over center until the driver pulls on the shift lever.
I wish I could take one apart and measure it. I’d bet weed burner would like to do the same. Without the engineering drawings you won’t know if it’s in spec, but most of the internals need to be chrysler sized to fit in the case.