Cut out the old rusted stuff, weld in new. That's the only way to fix it correctly. I had the same problem, I had to pay someone to repair it (about $400) to do the work, I don't have a welder yet.
I'd also recommend getting a steering box brace to help spread the load of the steering on more than one spot on the frame.
1989 Jeep Wrangler - Daily Driver and project - Red Jeep Club #216, NRA Jeep Club #5
1967 Chevelle SS396 - Main Project <--454, TH400, 4.10's, FAST
1982 Caprice Classic - Another project <-- 350/700R4, 4 wheel discs
1979 Honda Civic CVCC - Yet another project <--goal of 60 mpg