Rear seatbelt length
 

Bit of an odd one this..
But do classics have different length rear seat belts?

Reason I ask is that I have a JDM v3 and it's almost impossible to fit my sons car seat in without a couple of people and some brute force and swearing!
However in my father in laws UK v6 it goes in fine.

Iv checked the belt and buckle and they are all pulled out fully, as though the belt in my car needs to be another 2" longer.

If the uk or v5/6 belts are a bit longer are they a straight swap into a v3?

Thanks