yes there are rules for the IVA, but more to do with the position of the seatbelt/harness mounting points(upper ) in relation to the position of the seat & now i believe seats must have some form of head rest area so the old style short back seats wouldn,t pass etc.
As long as the seats are securely fixed i don,t think you,ll have a major prob with the regulations, after all its a safety point that you can see over the scuttle
andy