The IVA manual just seems to state it needs to have strong joints where it joins the chassis and corners of the cage, as well as drill and welded seatbelt anchoring points. And be designed to spread the loads throughout the chassis.

So doesn't look too hard to get sorted since i'd be paying a company to do it for me.

Thats what I read and thought do it later rather than have something else for them to look at and give greif over and fit it later.