It is true, you should always be careful when welding structural parts. But cast alloys aren't much different than billet alloys in terms of weld-ability. What is important is to use the correct amount of heat and the correct rod with the correct process (MIG, TIG, Stick, etc). Many people who weld aren't certified welders and have learned how to lay beads, but don't get proper penetration or use too much/not enough heat. That's when it's dangerous.
My suspicion is that the cast part will take less heat to weld than new material being welded to it. In those circumstances, I pre-heat with a torch the new material so it's already hot and I don't have to pour a lot of heat into the new joint (introducing weakness into the cast parts).
Have you pulled some measurements and confirmed the same as the other thread found? It's also possible that you could machine the ends of the pivot arm down 1/2" on either end. This might require some mods in the actually pivot tube portion of the swing arm, but again, if you could avoid the significant frame changes, this would be much easier in my opinion. Centering the wheel then becomes a bit of spacers made to relocate the wheel should you need to trim the pivot arm unequally.
