OR, the whole lot chopping out, and a Semi space frame Bolting in!

I'm generally after a bit more info on a rear-space-frame...
Once the rear is cut out and the frame bolted in, what do I need to do about getting it all boxed in?
Is it a case of boxing the rear up like a floor? Using sections of alloy plate.
How much am I looking at for frame and suspension, hubs and whatever else I may need?
Where can I get such things?
<|ben|>