Just a couple of simple 5 mm plates welded to the frame for me, a tried and tested method based on the dims Ashg provided a while ago.

i was chatting to my local iva man about mounting seats and belts and he was saying if the seat is movable (on runners) postion then the bottom seatbelts mounts need to rotate if mounted onto chassis to ajust angle to seat postion.
if the seat is fixed the belt mounts dont need to rotate.
so thay dont need to ajust.