cutts covered how to weld the diff like you wanted, and to give a little more of his advise, to strengthen things more in there, stick pieces of metal in there and weld them in there. Don't do this if your questioning your welding skills or machine.

A shackle flip would be a better idea for a lift over blocks.
What ever you do, don't weld blocks to the axle, and never stack blocks.

What do you plan on getting out of the lift? What size tires do you want to run?