The Mr2 seats require special attention when being built to mimic the 430 seats, the rollbar grp section prevents the seats going all the way back, take a look at some of the 430 builds at Stoneleigh.
I've spoken to the guy who's going to do my interior, between us we'll build the seats with this in mind, my job will be to alter the metal frame to accommodate the indent, the seat foam contour will then pad around that area.
D