If that side is rusted tha bad the other side s interior will be bad also. It is easier to replace the complete unitized front. It one time before my son took over the shop to do Engines we would replace the complete front in a week end. The easiest and the best way to do the job. When we were done you could not tell any welding was done on cars. There is a trick to doing them. And if they need a torsion bar cross member That is done separate from the font clip for a reason.
Nothing worse then Having a rail installed on a car and then having the other side done later. The hard part is finding fronts which I have here. The last one we did was mikes 72 Duster. drive it in on friday drive it out on monday. $800 labor. $400 for the front. No paint work. Just put one on for state inspection.