If I'm not to late, And the shift lever has not been removed from the top plate And you still are able to shift in all 4 gears except reverse. (that was long). I think the problem with reverse is the position of the of the "reverse shifter lever". It needs to positioned in the "reverse shifter head".
Laymens terms, The vertical flat rod on the passenger side of transmission that moves the reverse gear has to be positioned on the reverse shifter shaft mechanism.
Just move the vertical flat rod so it aligns with the reverse shifter shaft mechanism.
Good luck
