Peter - I assume the cardboard was set in position for the measurement?
(And just left laying flat in the photo.)
In which case, I think you have designed a way of matching the IVA measurement.
Not sure if I've posted this link before, or even if it covers seat belt mounting.
But it has a lot of practical demonstrations of the test and how to pass.
https://www.rodsnsods.co.uk/threads/...h-biva.405393/
Finally, the new welded plate is quite short (relative to the length of the original mounting bar).
So, I'd be tempted to bolt a cross brace bar between the two original pieces to form a triangle
just in case.
(Assuming it wouldn't be in the way of any other fitting.)
Good luck, Paul.