Jeff,
I had the same problem as I wanted navy belts to match my interior. To "correctly" mount the belts using the factory mounting points, you'll need some special ends for the belts:
The mount outlined in red attaches to the shoulder portion of the belt and is bolted to the two mounting bolts that you will find on the interior panel that covers the rear wheelarch. The mount outlined in yellow attaches to the floor between the seat and inner sill. There are mounting points already placed in the floor and covered by rubber plugs. The third point of the belt mounts to the driveshaft tunnel. There should be rubber plugs visible from underneath the car that you can pop out and insert the proper bolts.
Here's a link to John Sim's excellent site (bookmark it, you'll want to go back there in the future for tons of useful Healey stuff) that shows how the mounting points go:
https://www.healey6.com/bulletins/Seat%20Belt-ASJ.pdf
You can source the floor mounting fixtures and rear inner arch fixtures from David Nock at British Car Specialists:
https://www.britishcarspecialists.com/
You may wish to check if David can have a set of belts made up to your color specifications.
I bought my belts from
https://www.seatbeltplanet.com/ They are the online site for Beams seatbelts that were the original suppliers for the belts that were installed by BMC dealers in the U.S. They will happily make up a set of belts to your specification using the mounts you supply. I've worked with Steve Pekrul who has been extremely helpful and responsive:
Steve Pekrul , Senior Sales Manager
Tel Toll Free: (877)435-6363
stevepekrul@seatbeltplanet.com
Turnaround time is about 2 weeks.
If you decide to go this route, shoot me a pm as I have all the webbing lengths you'll need to provide.