Mike, yes you are correct,with the new belts i fitted ,takes up 2.3/4 at the rear,
I am 5ft 10inches so with the seat where i use it, there is still a 2inch gap before the seat belt roll.
I will show with a few photos, how to fit, not because i consider to be an expert, but the reason is,over the years
all the classics ive shipped back to NZ , Ausy, ,With there crazy laws, seat belts have to be changed to be able to ,pass
the safety law to drive on there roads.
Even my lovely 73 450SL, convertible had to change them, and you know how strong and safe they are.
In US theres plenty of choices, colours to buy,but safety you have to think about , always use factory mount holes wear possible,
use correct bolts, brackets,and backing plates , spring washers, a must, i changed my 230SL because old crappy, worn , original
still fitted.----will show second page.