If it was me fixing the manifold, I would grind the crack out a little bit so the weld has someplace to go and bond without just sitting ontop of the manifold. I think a 110 can go up to about 1/4 inch. I can remember one of my first welding jobs, I had to weld 3/16 pipe to 1/8 plate. . .and I didn’t have any problems. But then again it was a small part, and got hot very fast, where as a bigger part will take alittle while to get hot.