I would spend some time on the lower frame rails for sure. Maybe weld some strip along the bottom. Looks like it might be rusting from the inside out. I have a 89 frame that did that.  I never attempted to weld it. I'm not so great at tricky welds like that.

Ok i managed to find a few pics so i sort of know. It does seem its rusted from inside out as there is a few rust stains dipping out. How will i go about the rust inside? Maybe some sort of rust remover? I haven't got a clue lol