Cut the rust out completely and buy some tube steel the same size as the frame rails and weld it in or use some plate to make you're own repair,as long as it is all in font of that tube crossmember in the frame you'll be fine as far as structural integrity.Just welding plate over top of a rusted area will cause more problems down the road ,plus it will be a real PITA to try and weld rust.
Just remember to use a "fish plate" as some call it if you cut the whole front tube off and tie in a new one,it is a diamond shaped plate to go over where you tied the new section of frame in to add strength to the newly welded area..
You also need to check with you're local DOT inspections,or DMV..i see you are in Illinois,if i remember correctly they are pretty strict on any frame rail repairs as to who does it,like a state certified weldor/fabricator certified for frame work in the state..not sure..but best to check n see what they're laws are on the issue
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