do not do this it will set in the tinworm further...
Quoting 14u2nV:
I know it's not the best way, but if you want to get it repaired cheap, but long-lasting, you could wire-wheel/spot-blast that whole area as good as you can get, then fill the space with that expanding foam. Once it's dry, trim it down, then glass or bondo over it. Sand it smooth, paint it and you'll be good to go. I think that's the route I would probably take, as it's something you can do quick, with very few tools/skills and if you ever decided to have it done right later, it wouldn't be a big issue.
you'll need to get into the quarters to get at all the inner strengthening panels.. you'll be glad to know that the complete sills (middle inner and outer, and the pillars) are the same on non vr-4 and 2wd and vr4 cars. i had a pair of sills and about a foot of floor bulkhead and pillars for $10 a side here in the uk. if it's not got skirts on the donor then the sills should be very clean.
cut back to good metal, and overlap.. ie. if two panels join (pillar) splice/cross the joint, or inner panel in one place the cut and yoin the outer 3" away. you'll retain a strong shell for that.
Take off the door, and weld some angle diagonally across the openings to hold it all true as you chop the sill out. if you want the body manual sections jus shout..
Rich