https://www.wescoperformance.com/lb60.html


I installed these in the back seat of my 1967 Riley Elf..bought the hardware package and drilled through the rear seat panel.
You can add a steel plate in the trunk without welding. 
Comes in a variety of colours.
Also replaced the front lap belts with the three point front belts in Red.
Depending on the storage location of your Mini you might consider replacing the front belts too.
40+ year old belts are probably sun damaged and shredding.
Deb

The ones you bought are the lower sill mounts for the front seat belts.

I didn't know they were available like that either. 
Thanks 

Probably the ones I purchased are more suitble for early cars without seatbelt mountings.

RedRiley-that was an excellent answer. I had no idea Somerford Mini made those.

To paraphrase Obi Wan, I don't think those are the brackets you are looking for. Try these: Rear seat belt anchor kit

I looked here at our host first. Didn't find them even with a part #.  Ended up buying at MINISPORT. Part # MCR11.31.10.00
They are meant to be welded in. Besides the belts you'll need the mounting bolts. 
Second image is rear location for mounting. 
Because of close proximity of center mounting I may cut the brackets on a bias \ to move the holes closer.

If you can find a MkIII or MkIV from the CA market, they had rear fitted lap belts.  There were lots of these cars around and I imagine many found their way to the states and maybe in "pieces".  I imagine reinforcement at the mount areas would need to be addressed

I don't think our host stocks them, but the mounting points can be added. Welding is required.
LINK

My 76 Canadian mini has seat belts all around.

Oz minis of this vintage had factory fitted rear seatbelts.  Two box sections and reinforcing plates were added to the rear bulkhead/tray to make it strong enough to provide secure mounts for the top brackets.

I presume this mod (or something like it) was incorporated in later models by Leyland et al?  Maybe find a later car to replicate the mounts??

Straight lap belts are an easy DIY mod..

Cheers, Ian

take it to a body shop for the install.

Does anyone have any experience with adding rear seat belts?

My 76 Mini does not have any, nor any sign that there ever was any.

I would like to give the Grand kids rides, but really don't want to without seat belts

I see the host has a three point retractable available, but without anchor points identified, or hardware, I don't know where to begin.

Thanks for any information

Biggles