Do not worry about the the thickness of the material in the hatrack/parcel shelf area,it's actually thicker than that used in all later mercedes which had the baby seat bolts as standard( here,I don't know about the USA which doesn't have the same safety standards as us) . As I mentioned, the seat belt attachments are located there any way. A 10KG baby in a capsule isn't going to pull the bolt out in accident .
Lap belts are dangerous and were obsolete in cars 40 years ago. human bodies in an accident go underneath the lap belt or bend forward over the belt . This is why Mercedes installed 'lap and diagonal' belts from `1959. Strangely the American legislators didn't consider seat belts to be safety items until the 1980's.
As a matter of interest my Cousin Dr Simon Fraser Designed the original baby capsule when he was a designer at Porsche Design. It came about when my Cousin Daniel was born and he wasn't happy with the designs of child seats and baby capsules didn't exist.
Jeep Wranglers dont even begin to be considered a safe vehicle,being a truck.
heres the layout in a W111 Coupe,apart from the top of the front seat belts,the sedans are the same. you will find the threaded holes in place already.