The problem with welding aluminum is that it will destoy any heat treating properties of the alloy. Aluminum does not like being welded. Drilling a small hole at the point of the crack would reduce local stresses, but it would have to be at the head of the crack. Tough job. As this is a fatique induced crack, and a drilled hole would reduce the local stresses in that one area, it is likely that a crack will start some where else.
I am not a gunsmith. I am an engineer, and I am familiar with some aluminum alloys. Aluminum has always been very susceptible to fatique cracking.
What about getting an Al alloy frame and rebuilding? You have all the parts. Kimber sells frames. Their web page is
http://www.kimberamerica.com/ Their phone number is (914) 964-0742. They have a compact Al frame for $220.