Will a GEN III Hemi fit into a 71 Demon engine bay? Will stock suspension work?

It can be done with A-bodies, though I don't know of any Demons. I am familiar with one 67 Barracuda with this swap. There should be a lot of info if you search the forums. Not a trivial undertaking, but yes, you can use the stock suspension and k-frame, although I think a lot of people go to coil springs. TTI even makes headers for this swap. The main point of interference, as I understand it, is at the firewall.