Yup, I understand that. People want way too much these days for what is essentially garbage IMHO.
Was a time when anything with over 100,000 was difficult to even give away.
Once was anything ten years or older wouldn't fetch anything more than 3-5 grand, even in immaculate shape. Now guys think 20 year old trucks are worth anywhere from 5-9 grand.
Thats just nuts.
My K2500 6.5 TD with the "big" 8600 lb gvwr was purchased for 5 grand amost 4 years ago now and I thought I paid way too much. In retrospect I did pay too much because the engine had to be replaced a year later. Live and learn I guess...
I'd recommend when you plate the frame, make it a lot bigger than what you think you need. For example; that hole you have in the outer side I would probably plate from just in front of the cab support visable in the pic to back where the frame starts to sweep up in the back.
If you have access to a metal brake, i would bend up an angle the length of the plate and wrap it under the frame as well. The angle adds at least triple the strength a flat plate does.
Over kill?
A bit.
But steel is cheap and you'll never have to worry about that rail again.
Make sure whoever welds it is good and gets proper penetration. It's hard to tell from your previuos pic, but the plate already welded in looks like surface welds with poor penetration. The key to a lasting and reliable repair is the quality of the weld.
Cheers