1966 Valiant slant six standard transmission issue.

Another thing to look at are the rubber bushings on the end of the shifter rods. 41 year old rubber...well, you get the idea.
BTDT on my 64 Val 3-on-the-tree. When I got it running a year ago, the linkage kept jamming and I'd have to pull over, lift the hood, and manually unjam it from the top. There is an adjustment procedure in the factory service manual.