If you TIG it, do so after vapor blasting. The cleaner that material is, the better the outcome.
I run my machine at 50Hz and 65% balance when welding these old cast cases. Wash the area with the torch first to help draw any impurities out, then lay your first bead down. Peen it. Repeat. That gives you a much better edge to patch to. If you can find it locally down there, 5356 wire has been my preference on these cases, 2.4mm. I bump the frequency way up to weld the patch in, like +150Hz and balance at ~72% for better penetration and focus.