A doctor also wont give you a new heart because of a heart attack ,its always best to stick with the original..
If you change out all the panels just because they have surface rust on the inside you might as well give up now and buy one of those (pos) repop mustangs and put twice the money in it that it'll ever be worth or even ..and I'll tell ya something else ,that pillar with the surface rust inside would most likely out last any recycled rust panel from China.
Dipping it would be best but very expensive. usualy a good dip would consist of a first dip to remove all paint even inside welded seams...a second dip in Phosphric acid (just like the factory does) then third ,a dip in epoxy.
Ospho is great product but not a magic cure and might be your only option.
I use a lot of it and trust it completely.If you decide to go this route let me know and I'll give you a few tips....meanwhile...a few pics would help