The 3.90 and 3.91 would have no appreciable diference between them as they all have 1:1 4th gear ratos. But the 3.73 is noticably quicker off the line.
I had thought...and seen that originally the 3.90 ratios were for wagons...but after finding a couple in late 411 sedans...as well as 3.73's in 411's and 412's.....I can find no real reason behind the use of one or the other.
With the much greater available torque of a 2056.....I would think that you would probably not be able to really see that much of a difference between either one.
You could get a glimmer of it though through simple rpm and math. I can't remember the ratios off the top of my head....but multiply 1st gear by engine rpm and then by the final and see how many revs you get. The difference is more in a lessening of work to the engine....which may not be that notciable on a 2056...but was readily seen on a stock 1.7. Ray