Most common cause is worn clutching teeth on the 4th (input shaft) and the 3-4 slider. I would first check the obvious shifter linkage adjustments but if it goes in solid but pops out as soon as you let of the gas it probably needs a new input and slider twocents
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64 Plymouth Savoy
493 Indy EZ's by Nick at Compu-Flow
5-Speed Richmond faceplate Liberty box
Dana 60