Transfering from a 70mm print would not be ideal it would print in extra grain not on the original 35mm negative printed in at the blow up stage.
I wonder what the state of the internegative used to make those willits deigns film cels is in? The empire cels had good color and grain structure and contrast, the star wars ones while going a little magenta werr far less faded than they should have been given the territory of eastman filmstock before LPP. But the color on return of the jedi was almost gone and was faded severly, funny as the same year LPP printing really took off jedi was released. Not sure if these represent how the 70mm film cels actually look, or if it is due to the process of how they were scanned but they are here at this link.
http://www.jedi1.net/
Ideally what you would want to do is retransfer the vistavision plates or elements from their original camera negatives in 8 k, not the 35mm reduction of the same elements. The you would also transfer the live action 35mm footage at 4 minimal. The Vistavision effects negative was i believe 65mm.
Which is what i think Ridley scott did on blade runner they had saved the original opticals produced in vistavision, and were able to use them for that added sharpness and effect on the blu ray.
Star Trek the motion picture was also photographed as far as its optical effects n vistavison by John Dykstra but Paramount would not pay the expense to retransfer them. I wonder if the seperate elements were even saved, knowing them probably not as they never saved the optical elements for the original tv series making it necessary to recreate all the effects in cgi,lame.
Fortunately for star wars fans as lucas has said in the 1997 restoration documentary he saved every piece of film and all the separate opticals.