I bought a pair of 'Securon' E marked inertia reel belts from Holden (not the cheapest company but convenient to me) they were about £40 each inc VAT. I had thought about fitting harnesses, but decided against. These belts are pretty basic but OK & easy to install, I expect you can find cheaper & I am sure you can find better for more money. You may need the angle adjustable version for 2 door cars without door pillars which are about £10 more, I had used my roll bar for the upper mounting.
Yes, I have MODIFIED my interior with fixed Securon belts.
I'm not a fan of inertia reels (another mechanism to go wrong), and with the seat right back there's nowhere much to hide the reel, unless you mount it on the parcel shelf, MGBGT style. And the MGB ones get awkward and won't spool if they sense that the reel is not level.
My mount points are 8mm plate and are wedged and welded inside the tapered sections, so they are not coming out, ever. See the website for pictures, although you couldn't really do this unless the body was off, you'd be in danger of trying to weld the plate to the petrol tank.
I needed the long (45cm) stalk version because the strong point is a long way back, and there's a lot of me to go round.
Sometimes Europa Spares is cheaper than Holden, although they don't keep the same range of period Lucas stuff. They do Mail Order as well.
How did you attach the rear belts? I've got fixed ones in the front, but have been puzzling over how to link the rear attachments back to something strong.