jjonesnc I doubt that it is "weld or die" and I think (given the small amount of evidence I have) that you could travel now and fix later.
Is the pic of the frame or of a cross member?
It looks like the belly is still on. Did you just stick the camera down thru a hole from above?
Assuming the rust out is from the water leak you have ID'ed, the breadth of it may be confined to the area you pictured, (We Hope).
You will really never know until you open up the belly. (Not a small job)(but not monumental either).
Assuming you only have upper access right now, then you will not be able to wire-brush much rust, Only one side of frame perhaps.
Here is what I would do. Brush what you can, but dont remove it down to bare metal. You need a layer of rust for this solution.
Buy some OSPHO. (call a sherwin-williams store or other paint store. Lowes sells a similar product (different name) but I dont know its success). OSPHO is very liquid, it is an acid that chemically reacts with iron oxide and changes it to iron phosphate. But you must have rust (iron oxide) present to effect the reaction. Rig up some method , mustard squeeze bottle, pump up sprayer, sling-shot
, whatever. And saturate the area of frame/outrigger. This will buy you at least 6-8 weeks of sound sleep. OSPHO after the reaction, acts as an ideal primer, but you might not want to paint over it until you get a better idea of what is going on under there. The rust will eventually come back in spite of the OSPHO if conditions allow.
OSPHO is mildly dangerous and minor precautions need to be followed, I have had it on my hands and no problem if washed off fairly soon. Last I bought was around 20 dollars a gallon. The Lowes stuff comes in quarts. If I find name of that product I'll edit and add it to this post.(EDIT: Jasco Prep & Primer 6.00 a quart).
OSPHO by the way is heavily used along the coasts in the offshore oil and plant maintenance and painting industries. Great stuff.