I found this procedure from a Link about aftermarket suspsnsion systems crenweldge had posted earlier. Not exactly applicable but you can gather some great tips and basic info from it for your application.
http://www.watsonsuspensions.com/CMSPages/GetFile.aspx?guid=ac292487-3336-4100-abf2-a1dffd55d04d

As 3B suspects, and I'm pretty sure, you aren't welding anything at all "cast iron". The Dana axle tubes are steel and the spring perches are either forged steel or cast steel, in any case easily weldable (as you can see they were welded on to start with).

I strongly suggest you remove and reattach the original spring perches, that will give you a more professional and likely more reliable solution. An added benefit of welding the spring perches on where you like is you can measure, adjust, and fix any issues you may have had with pinion angle. Here is where you start to learn about that part.
http://www.stovebolt.com/techtips/driveline/setup/index.html

Grigg


•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-