OK, I said I was giving up, but I just can't let a smog manifold go unrepaired
So was doing some other tests and welding different cast iron stuff.
The "special" nickel wire was just not sticking to the parent metal. So in the "what the heck" moment, I just used the regular .025 mild steel wire. It seems to stick much better and don't see a join crack (yet).
So I went ahead and did the smaller of the cracks on the smog manifold. Its heat and clean 1st. Then when warm to touch, small beads, and grinding, then another bead and grinding, etc, waiting for it to be warm to touch each time.
Seems to stick properly. Finished it up with some die grinding and media blast. I will let this sit over night and then heat it up with a torch to see if any cracks appear.
Not sure why I am getting the voids, I ground them out between welds. I found after the first beads that if I start the weld on a grinded bead it flowed to the cast iron better than starting on the iron. I will work on the mating surface more if and when I do the 2nd crack. That will get a gasket anyways.
Opinions welcome as always