Steering 64 Signet..

It has been done, but is very difficult. Hardest is providing proper structural support for the rack. Read on rodder sites like HAMB. Watch the Wheeler Dealers episode where they fixed a hot rod that was almost un-driveable due to bump steer. Ed China gives a simple explanation of the problem.

I can see your need to drop the Mopar gearbox for space, though a manual box often solves that. Otherwise, nothing wrong with Pitman arm steering and after adding the new structure, a rack & pinion might be heavier. My 80's M-B cars have Pitman arms, as do many U.S. trucks. Consider the new electric-assist steering like Chevy Cobalt or Volt. The rack is then manual and smaller/simpler, putting more bulk under the dash where you have room.