I already answered that question. Any design process or study model building would have been done after the production told ILM what they wanted.
This may not be the case because it's all too obvious that except for the Grissom
design studies were rejected
by Nimoy/Bennett/Winter because they did not get the design studies they wanted
(and that's not the kind of stuff ILM would write home about).
Either ILM thought they were doing models for a prequel (blame Nimoy/Bennett/Winter) or Nimoy/Bennett/Winter didn't like the apparent retro style ILM was aiming for (blame ILM for their lack of vision what the next gen of a Federation starship should look like).
As it is obvious from the Rodis storyboard of the revised four-nacelled Excelsior
viewed from the spacedock lounge (not the Enterprise
!) the warp nacelles were apparently what the ST III producers found most fault with.