A very common issue is that, if just slightly out of adjustment (from being worn, perhaps?), the transmission safety interlock will not allow the linkage to move.
Get underneath and manually shift both shift arms (the parts bolted directly to the transmission) into their "neutral" positions. You can easily feel and hear the clicks when they're in their mid-positions.
Once there, you should be able to shift into any gear. At least once.
One (the rear one I think?) is for the 1st-Rev shift, the other for 2nd-3rd shift.
If you get them in neutral and can then shift into first or reverse, you'll need to re-adjust the linkage to get things to work together again.
If you need to adjust things (and you might as well anyway, just for the heck of it), start by loosening the nuts holding the shift rods to the shift arms. Make sure that the shift arms are still in their neutral positions.
Manipulate the shift rod levers (on the column) into their centered positions, either by hand, or by using the column shifter if you prefer.
Insert a 3/16" diameter pin (or a drill bit?) into the alignment hole through the center of the two levers at the base of the column.
Re-tighten the two nuts, and you should be good to go.
The exception to the "good-to-go" scenario is if you have a lot of worn out parts. The bushings at the tops of each shift rod, where they attach to the column's arms, can wear out, or even be missing. If so, new ones should be easily available.
Good luck, Hope this helps. Common problem with old column shifters.
Paul