Hi Dave,
Thanks for the message. I got the term 'bottom chassis' (and the idea of swapping it) from this individual who builds them from repro steel:
http://www.musclecarmetal.com/WIP/Bottom/index.htm
Take a look at the pictures, it will make more sense than me trying to explain it. A big job? Yes. But if you add up all of the metal you would have to buy and weld together, it's over $2,000. So carefully 'harvesting' the bottom chassis from the donor body was the route I was going to take. Hopefully this makes sense.