From what I have seen in the past the use of 4 brackets with 50mm spaced holes to fit to the original mounts have been used to move the standard fitment seats back an up. then it is a matter of filling the gap underneath to support the weight so as not to break the mounting points.
As a non welder I used the blue modeling strofoam in the past as it is very light, easy to cut and shape. It is also strong enough to stand on
http://www.ebay.co.uk/itm/50mm-EXTRU...3D111672221476