1966 Valiant slant six standard transmission issue.

Its a backwards N pattern A903 (A230 was later) IIRC. It doesnt have a synchro in first so you have to be stopped. Its probably a linkage issue. Go under and look at the linkage and cycle through first gear. Remove the 1st gear linkage from the shifter and see if it can engage the trans more than what the rod will give you. If so, then you can adjust the throw. My buddies 4sp did that and he just held it in first as he was never in first more than about 3 seconds.