I just removed some inertia type rear belts from a 60's Cooper for a customer and they were fitted thus
Inertia reel bolted to rear parcel shelf either end with a thick captive nut plate on the underside, the lower point was bolted to the bottom of the arch tub, again using a thick captive nut plate and then the buckle part was bolted through the rear bulkhead through the existing off centre holes where they would be bolted in a later car.
That was ok but I would have personally made up some larger spreader plates and welded them to the mounting points.
If I was doing one now, I would certainly follow the method used in the later injection cars but as has been said, this would involve fitting the internal C pillar reinforces, the reinforcing plates inside the boot on the back of the rear bulkhead and the mounting brackets on the arch tubs which would void any originality but once trimmed, these brackets would be hidden from view anyway.
Here is what you would need.
http://minispares.co...|Back to search
http://minispares.co...18.aspx|Back to
http://minispares.co...20.aspx|Back to
https://minispares.c...|Back to search
https://minispares.c...|Back to search
I can't seem to find any availability on the inertia reel mount but it's pictured here http://www.somerford...roducts_id=1222 That may be something that needs to be sourced second hand perhaps.
I fitted arch tubs to an MPI as part of some restoration work and have a photo of the inertia bracket which I transferred over from the old tubs

Cheers
Ben
Edited by Ben_O, 09 January 2019 - 01:52 PM.