[craggle]

As long as the engine and gearbox are in the same location in the De-Dion as the standard chassis then the following applies.

Turn the gearbox plate back around so the lever is nearer the back of the gearbox. It will be too close to the dashboard the other way round.
Fit the Dax lever facing forwards on the passengers side of the lever in the gearbox. This seems to place the gear lever almost lever with the steering wheel and dead central on the transmission tunnel.

All I did to mark the hole is to remove the Dax gear lever then measure from the face of the firewall where the tunnel bolts too back to the centre of the gear lever position. Drill a hole and try it for size. Enlarge this hole until you can fit the gear lever and still select all gears. I left about 1/2 clearance between all positions of the lever and the tunnel.

Craig.