70s ground floor staircase, but only 69cm ceiling beam spacing

Good evening.

I could get a new attic ladder with the dimensions 70x120 cheaply. However, my ceiling joists at the place where it is supposed to go only have a width of 69cm.
Could each of the two ceiling joists be tapered by 7-8mm over a length of 120cm, or should I rather buy a different attic ladder, e.g. 60x112cm. Unfortunately, that would cost three times as much.

You are talking about a retractable attic ladder? They are usually a bit narrower than the specified size, and the gap is filled with an insulating material. Just refer to the instructions regarding the exact measurements. It could fit anyway. Otherwise, nothing can be said in general about tapering the beams. It depends on how strong they are in their original state and what percentage you want to reduce.

How wide are the beams?