Make sure the gearshift levers are squared off, as well as the holes in the operating levers.
They wear each other out.
Mine would get stuck in second on a brand new shifter because the gearshift levers were worn, no longer squared off.
1/2 lever would put it into 2nd but not take it completely out.
Sent mine off to Brewers. New gearshift levers are not only squared as heck but deeper than originals, more closely matching the depth of the operating levers for more/better engagement.
1966-70 SIDE COVER 1-2 GEARSHIFT LEVER REBUILT -EXCHANGE ONLY- Mopar A833 4-Speed Transmission & Component Specialists