I did own a 1990 Talbot Express, which would have had the same box, and gear changing mechanism. The gear selection has been described by others as difficult. I particularly remember problems in changing down into second gear when approaching a roundabout, and going downhill. I have some vague and brief notes from that time.
From Peugeot workshop manual I have noted Short Link 111 +/- 1.5mm, Long Link 159 +/- 1.5mm. These are factory settings, but the vague bit is thatI have failed to record where the measurements are taken. They do relate to adjustments at the box. I have recorded that when engaging second gear the selector shaft moved further into the box. Also "Short link pushes to move selector out, hence shorten short link by one turn.). As I recall some improvement was obtained.
Perhaps decoding the above, and applying to your specific problem may help.