Wenn wir nur von den Rücksitzen und Gurten sprechen, scheinen sie mit zwei Schrauben zu passen.
Talking about just the rear seats and belts, they seem to fit by adding 2 screws
Ok, switching to English, because I think my point is not being made clearly.
There is a reinforcing element -- thick metal -- under each seat/seatbelt attachment in every car. That is because the forces on a seat or seatbelt in a crash are very large, thousands of kilos.
Because the HJ was made without rear seats, it may be missing these reinforcements. Martin at BigJimny describes the reinforcing elements in the bodyshell that are missing. You will see them if you compare a GJ and an HJ body shell. You need to do this.
Putting a rivnut through sheet metal, like the person in that other thread here, is not the same as having an attachment point. Sheet metal simply will not hold the forces on a seat/seatbelt. You could pry out those rivnuts with a screwdriver, they will not stand in a crash.
I have deliberately not referred to that other thread on this forum here, because that person is 1) doing something he has acknowledged is completely illegal, and 2) is doing it in a very unsafe way. He knows perfectly well that what he is doing would not pass an engineering inspection, because it does not include some important elements. He says he is going to remove the seats before his next technical inspection; I wonder if he is planning to remove the seats before his next car accident. Will his children be sitting in them?
You can add some nuts and the seats will "fit." But that is not the same as the original, reinforced, safe installation.
Of course you are free to do what you want. But you asked for advice. My advice is, follow "step 2" from my post above.
I assume you are putting in seats, because someone will be sitting there. Please, think about that.
This will be my last comment for you, since you asked for advice and then argued against everyone who gave you advice. Good luck to you. Hope you don't kill anyone you care about. 
