What a great idea to help with those "lengthy" journeys.
You could do that in several ways, but I think that it would be best to drill out the spot-welds for the seat runners and then to weld through the drilled holes after you have moved them backwards by the required amount.
You can also buy brand new runners and better (well..allegedly) seats.
Under the floor there are strengtheners beneath the runners and these may also need moving, as well as some reshaping of the floor to give enough flat area on which to weld the runners.