Just use the Sierra runners
I've just sorted out my runners.
You can simply use the runners from the Sierra passenger seat.
Take them off the seat and remove the brackets (angle grinder). You'll have very flat runners, linked with a steel wire. Weld two pieces of flat strip (80 mmwide) between the main chassis beams and you can mount the runners flat above the bottom.
It takes an additional construction between the runners and the seat but you can adapt this to any seat of your choice and decide on height and tilt of the seat later. Another advantage : changing to another seat is always possible.