You might get some more detailed advice if you can provide a bit more information. Is it heat treated already? Did you drill holes in the tang? Do you have a grinder? How did it get thinner in the first place, and how much thinner is it? Do you have a picture?
I've messed up blades in similar ways and will probably do it again. If you're a beginner, the tapered tang might be tricky to pull off, it certainly would be for me. Depending on whether you've drilled pin holes already, a hidden tang could be another option, but that makes the handle construction a bit more complicated. Another good option, especially if the gap is not too big, might be to use black handle material (e.g., micarta or G10) and use epoxy dyed black to fill the gap. Call it a learning experience and do better on the next knife. I have a drawer full of learning experiences...
