The way Ferdinand Porsche designed these, there is no need to do what you describe.
Just use the four wide, flat torsion leaves. There is enought threads on the snub nuts in the adjuster and the trailing arms to tighten it all back up.
Remember this design was well thought out to be for many applications that required different spring rates. The rates were changed buy the number of leaves used.
NEVER heat torsion bars, torsion leaves, torsion bars, coil springs or leaf springs by welding them!!!! Not only will the rate change, the area becomes brittle and will evantually snap.
Just a suggestion, sand, clean and paint the new adjustable beam with some decent paint. The cheap paint on it comes right off and they rust up quickly. This is aftermarket stuff made in Brazil or Mexico, not OEM German stuff.
You are going to be very happy with what you’ve done. Your car will handle and ride much better now…not to mention be safer.