The axle was placed on axle stands and the wheels removed to give better access. The chassis was then supported with wooden blocks hammered between the bump stops and the axle. The exhaust tail pipe support was unbolted and the top bolts were removed from the rear spring shackles. Lots of measurements were made and the top, inside and bottom of the chassis rails cut through with an angle grinder.
The wiring harness was kept away from the cutting disk using a piece of stiff wire inserted through the slot (the top edge having been cut first).
Angle strips were clamped to the rear tub supports using mole grips. The rear end of these lined up with the body mounting tabs on the cross member as an additional alignment guide. Then the cross member was supported using scissor jacks on piles of wood blocks, the outer edges of the chassis rails were cut and the old rear quarter removed.
In the photo below you can see a piece of wire attached to the wiring harness which was used to push it up the chassis rail away from the heat of welding and then to pull it back through the hole in the top of the new rail.