I say go for it, there's lots of rust, but if you're going to build something wild and crazy of it, you're at least not bound to try and make it look stock, which makes things much more simple.
With a full cage on top of that, you might get away with not doing all the rockers perfect and so on.
The shell is so far gone, that if it ever rolls on the street again (in any form) it'll still be way better than just crushing the car
You don't have any possibilities of sandblasting the shell yourself?
I borrowed a big air compressor and a little sandblasting machine (cheap portable sand pot), bought alot of sand and did it myself. It was very inexpensive, but I still have sand coming out of my ears..