What about the recaros in the mk2?
They have harness slots and after having had 2 in bits I'd say they are pretty flimsy.
Bolting the harness shoulder straps to the rear seat belt points means when you're weight is thrown forward the shoulder straps will pull down compression the spine.
And to be fair,the cars in a really bad state,I would expect some injuries from multiple impacts like that car has had.