I like your thinking.
On the tunnel, mark the required position for the bolt hole(s) one each belt.
Transfer this position roughly to underneath, where presumably the propshaft runs.
Make sure there are no brake pipes or wires in the locality. Drill a small pilot hole and check.
Don't know what size your bracket threads are but presume M10 fine? I'd drill a hole same size as the bolt, hold the plate with the nut under where the prop is, bolt through the belt, and tighten things up.
You will need Octopus arms though.
In my eyes that's good enough but I may add a bigger spreader plate to be sure. I don't see it needs welding for safety, but if it needs inspecting then possibly.
Now the dot,
I'd consider drawing the 'square' directly over the dot, and right up to the edge. On the top side.
I'd make a plate 30mm? wide, length equal to the depth of the slot inside the box section top to bottom. thickness. 12 mm.
I'd leave it long for a start. Make the square so the 30xdepthx 12mm plate just drops through, no gaps.
Pilot the dot, drop the plate in, spot the plate through the hole. Drill and tap the plate to suit seat belt bolt thread. Open the spot to bolt size.
Drop the plate in, fit the seat belt bolt, trim the top of the plate level.
It's then easy to weld round the top of the thick plate. If you bias into the thick stuff it will blend into the thin stuff and being on a top surface is easy to blend in anyway. Then remove bolt and fit belt.
If you drill a hole under the spot, you can spot weld the bottom of the plate too.
It's long winded writing and probably difficult to understand me but in my mind it's easily possible to achieve a solid mount.