Trying to sweep an extrusion with such a 'height' and profile difference in that small a gap will be a nightmare. You can make guide curves to for the sweep to better control this and maybe come up with a result that doesn't error out or create broken geometry, but it's going to look very odd.
What do you need to create this for? Are you making renders of this item with the weld blends? In other words....do you really need to create a crazy sweep protrusion in a tiny area to simulate something that might just be for a note on a process sheet?
Agree with Dylan here.....seems like a massive effort to get the tiniest gap closed for welding. The real world weld would actually fuse with and encompass more of the surrounding material anyway, so will not look like that little blend.