You could disconnect the linkage rods at the steering column by the firewall.
Then see if it shifts, if it shifts with the linkage rods disconnected, the problem could be the linkage rods needs to be adjusted.
1954 Chevrolet shop manual
Steering Column Gearshift Adjustment There are two levers on the transmission, one lever is for 1st and reverse gears and one lever is for 2nd and 3rd.
Each lever has three positions:
1st, neutral, reverse
2nd , neutral, 3rd
In order to shift, one of the two levers needs to be in neutral.
If you can't shift with rods disconnected the the problem could be in the Steering Column Gearshift control mechanism.
1929 - 1954 Master Parts and Accessories Catalog
1948-54 Pass., Comm., Steering Column Gearshift
