[Killer Bee]

had a three on the tree in a '68 C20 daily driver back in the early 90's..

two things I remember distinctively;

popped out of third under load or freeway speeds so I hooked a bungee cord under the edge of the seat over the shifter when my arm got tired

and every once in a while the shift linkage would get crossed up on the column at the firewall and I would have to go underhood and un-foul it..

follow the manual and verify all your linkage alignments and you'll nail it down..

good luck!