You had the oil pressure sender location welded shut, and now it's leaking? You can't have the place that did it reweld it? I'm not sure what you would want to try. You could try to seal it with something like Permatex or JB weld, but I don't know how either of those would hold up. If the leak is small, will little pressure, I think it would hold ok. A better solution, if you don't need to use that hole for an oil pressure sender, might be to have it drilled out and tapped to accept a plug. You could then screw the plug in with some Permatex on its threads, and that would hold fine. First, I would go back to the place that did the welding and have them redo it. If it still leaks, I would try to seal it with either Permatex or JB Weld. If that doesn't work, I would try to have a new threaded insert put in, or drill it out and tap it to accept a plug. Of course, you'll need to weigh the costs, and figure out whether these options will be cost-effective. B16A blocks are pretty easy to find, so if you can get one for cheap, yoo may want to swap blocks. Your B17 rotating assembly should also fit in a B16A block, so you can swap it over if you want.