I just installed the same set of bars from PST in my 74 Duster last weekend. I didn't compare the measurement to the old bars, but the new bars slid right in with no issues. Are you confident there is nothing in the LCA hex socket that is not allowing them to slide all the way in? I did need to lightly tap on the back end of the bars to seat them properly. After installation my bars are definitely more than 3 mm past the clip so I don't think that minimal length difference should affect you anyway.
Cley
