I've seen quite a few frame and suspension parts that are welded and have worked well for many years. I think the real concern welding suspension parts is doing it right, not that its been done.
However the suspension brackets we are talking about are all cast iron, and welding them to the mild steel frame does not seem like a good idea. If you have to ask I'd suggest you don't know enough to do it right.
As for the running board brackets, my concern regarding welding them to the frame would be the absense of any flex in a weld compared mounting it with a rivit. Over the length of a running board you will have enough flex to cause problems.
My Corvette, Subaru and K3500 all have welded suspension components. All three have more than 100,000 miles with no suspension failures. Done right its a very workable approach as proven I think by these three factory built vehicles.
But here I'm inclinded to agree with the guy who has a welded straight axle under the front of his truck, and say rivits as the manufacturer intended are probably your best bet.
And I think the best way to get them is to find another frame. They aren't that difficult to find and often very cheap to buy. I'm planning to collect one for free this weekend. You can't have it, but it proves they can be found. It won't be the first one I've dragged home for little or nothing.
