Depends on if it's an early car or not - I think mid '67 (by '68 for a fact) they went to collapsible steering colums and the pitman arms were bigger - I tried to swap the '67 arm to my new '68 box (for the collapsible steering column) and it was too small. If you have a solid steering column, its the small one - if it has a rag-joint and collapsible column, its the big one, I can almost guarantee you.