Don Elliott
Obi Wan
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Hi Guest!
smilie in place of the real @
Pretty Please - add it to our Events forum(s) and add to the calendar! >> Here's How << 
Here's the template I used. It's from Triumph in 1962.
Bill
https://s258.photobucket.com/user/TR3driver/media/Manual pages/tr3seatbeltanchors.jpg.html
Some time ago, I scanned and ".PDF'ed" some drawings/templates from a Standard-Triumph Service Bulletin of 1962, detailing suggested installation of seat belt mounting points for a TR3A. (At the time, NY, CT and possibly other states required at least installed mounts for seat belts in new cars.)
Here is that scan for installation of belts in the TR3A. I assume no responsibility or liability for these; I'm only passing along a "factory" suggestion!
... If I had to do it again, I would figure out a way to put 3-point belts in... You can't be careful enough.