Hi Rob,
If i were you i would not learn to weld on this part of the car, your rear frame actually does quite alot, needs to be strong, and as someone else mentioned, you may get a mig weld "looking" good but it may only be sitting on the top of the material and have no penatration at all, and it can just fall off!
You have gone to alot of trouble so far with all that trick design work, the machining of the half round bush blocks etc etc, dont ruin it all now with some crappy welding mate. Get it welded by a professional.
Spend some time learning/practicing to weld on a bunch of other rubbish parts etc, so that you can do it all yourself eventually. But dont comprimise your nice rear cradel.
Clint T