Welcome to the forum, your truck is really beautiful. If you're still running the original 3 speed column shifter, it's probably going to need some measure of rebuilding. They didn't work very well when new and nearly 50 years of wear makes them sloppy or non-functional. If the transmission is in good shape a Hurst shifter would solve the shifting problems, but you may not want to cut the floor especially if you are considering better transmission options available today. If you want to keep the original column shifter here are some links that may help:
This is the link that notsolo provided earlier, in a clickable version. It shows how to adjust the column linkage:
http://67-72chevytrucks.com/vboard/s...d.php?t=215307
There are many other threads on column shifters, but this one has a lot of information exchange and photos on rebuilding that might prove helpful:
http://67-72chevytrucks.com/vboard/s...column+shifter