Welding on a unibody

[L_O_O_M]

Hi guys, I was wondering if you can please help me with this. I have been searching all over but just can't seam to find the answer I'm looking for. I am looking into getting a old w108 Mercedes Benz as a project car so I'm doing some research into doing a rear end swap and replacing the swing arm rear end with IRS but that requires that I build a custom sub frame to mount the axle carrier to. Now I myself am not a fabricator but my friend is so he can help me fabricate,weld...etc. If you take a look at the picture I have attached you will see the rear end underside of the car I want to get,as you can see on the left hand side is a mounting point on the cars sub frame where one of the control arms mounts to. What I would like to know is can you weld your own sub frame to that sub frame as well as to the under side of the car? will it be structurally strong enough to mount an axle carrier too? not only that but in general can you attach your own sub frame to a car by weld your sub frame to the car's sub frame as well as the sheet metal of the body? From what I have read when weld to a unibody you are suppose to stitch weld, is that correct ?

Sorry if the question is a little confusing