1968 coupe rusted rail issue

Ok so I found a slight issue with the weep hole for my front frame rail rusting out over time that I negelected to find during my initial inspection.Upon giving my car a decent once over after getting her home I started poking around more on the underside and found one weep hole slightly larger then the others and as I proceeded to lightly prode with my finger I found it not only was full of debris including some old seeds from what I would guess was a mouses home but it also fell apart fairly easily up to the point shown in my photo. I did not probe much more then seen in my profile photo but it did open slightly more then this. I have not tried digging in any more with a screwdriver as of yet. Have not had time to put her up on stands and the plan was for this fall to do so and start cleaning the underside and patching a small hole in the already replaced rear floor pan. Fronts were replaced years ago and obviously they never did anything to this.
So where do I begin? My hope was not to strip it down totally as the interior is in really good shape. Was planning on cleaning up the underside and undercoating it after minor patchwork and replacement of a rear quarter panel. I will get some better photo's up this weekend by curious as to options as of right now?? As well I am new here and have yet to figure out how to post my photo on my thread so sorry you have to look at it in my profile.