Art Morrison does custom builds, I am sure they could build a full chassis to fit an A-Body.
Also Roadster shop does custom Chassis ,full frame stuff that is real nice. They make them for 66 nova's , which would be close to an A-body in size:
Most people don't want to get into that much fab work, usually removing the complete firewall and floor to install.
That is why you see bolt in suspension kits for the mopars , like the RMS or Magnum force transformer . They are a bolt in modern suspenions that provide handling without all the fab work