Based on my interweb readings, the "you can't get there from here" response was expected using my small MIG. Note that I didn't say it was a good welding question. As such, I am refocusing on oxy-acetylene welding. Otherwise, there is a custom motorcycle shop I have a lead on to talk with about welding the upper tang.
I am confident that enough heat can be applied with an oxy-acetylene torch to melt the fork tang into a blob. The upper tang is critical, the lower not so much. The upper tang is also thicker than the lower, so the lower is my target. But, as a non-urgent and non-critical project I will take some time before attempting any welding and destroy the fork.
This gives me a good excuse for practicing gas welding aluminum. Back in the day, all sorts of aluminum castings and parts were gas welded with most of the books published before 1940. So, with some oxy-actetylene books (including one by Union Carbide from the '60's) and a few how-to DVD's, off I go!