A swather tractor has hydraulically driven wheels at a cab end and castor wheels at an engine end. It can be driven cab forward in a working mode with a header on the forward end. It is rotated to engine forward in the transport position for more stable higher speed travel. There is provided a float module with adjustable and calibrated springs connected between a mounting assembly on the tractor and pivotal support arms of the float assembly where the mounting assembly can be raised and lowered by a lift system on the tractor. This allows the header to float relative to the lift system on the tractor. The float force of the springs is adjusted by raising and lowering the lift system in dependence on an angle measurement of the lift position of the header as it floats relative to the mounting assembly.