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Is this a stock, GM shifter? If so, I'd check the condition of the motor and trans mounts. If they're in poor condition, they could be allowing the engine to "rock" and cause the shifter to bind.
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You need to adust your clutch. the muncie 4 speed has all the gears moving at the same time, the synchros allow the gears to line up so that it will go into gear eaily with out grinding, but if it doesnt go in at all, sometimes it is just the clutch. I would start there since you say the linkage is adjusted properly and in good order. if there is a lot of slop, just adjust the clutch linkage so it is minimal