Hey picko,
For mine I got 2 pieces of 75 x 50mm RHS & cut 160mm long, welded together & machined a 76 diameter hole through the centre of the join.
Then it's a matter of welding some end caps on (modification approval requirements), setting the pinion angle & welding to the axle housings.
I went for the 75 x 50mm to lower the rear.
A lot of blokes use 50 x 50mm then add lowering blocks later on.
160mm is a bit longer than what most people have done too.... This was an effort to reduce axle tramp.
Here's a photo of mine prior to welding.
