Sandman

The front cross member on my frame (where the frame horns join) is rusting out. A long time ago I installed an aluminum cow catcher and I guess the different metals got too comfy and started rotting. The rest of the frame is great - can I just cut this piece off and re-weld some thick square stock in there? Anybody face this before? (90 4runner).

InternetRoadkill

A solid piece of iron pipe or square tube would prolly work rather well. Just get the spacing correct. You may want to put some end plates on it if the new tube doesn't match the frame edges exactly.

gonzo

If your talking about the crossmember under the radiator yes. I have done that using 1/4"x 2" box steel.
Good luck