The C6 gear ratios..
1st - 2.46
2nd - 1.46
3rd - 1.0
The Ford E4OD / 4R100 overdrive trans has a very similar gear train/planetary.
The E4OD / 4R100 ratios...
1st - 2.72
2nd - 1.54
3rd - 1.0
OD - 0.71
The original E4OD / 4R100 gear train (minus the overdrive) can be swapped into the C6 with minor machining.
These days there are vendors that sell a "low gear" or "wide ratio" (same thing) gear set for the C6 based on the E4OD gear train.
There may be custom options from said vendors.
1st gear difference is approx 11%
2nd gear difference is approx 9%
In drag racing timed runs there will be a measurable ET difference between the gear sets.
I have a 66 Galaxie with 390 and C6, and have the E4OD / 4R100 gear train in it.
You can't tell the difference in normal street driving.
Oh yes, maybe a little more punch on launch... but really hard to tell without timing it.
"seat of the pants" feel on the launch.... nah.
But I have a very healthy 390... so maybe that's why... (shrug)
For the record, my Galaxie had 3.70 gears that the previous owner installed, and I went back to stock 3.00 gears, with the wide ratio trans.
there was never a C6 option from the factory with the wide ratio / low ratio gear set.
ALL C6's came with the ratios listed above. 2.46 and 1.46
Ford Racing even published a bulletin for installing the E4OD gear set in a C6.
"c6 wide ratio gear set"
C6 Low Gear Planetary - TCIŽ Auto
Note that the E4OD and 4R100 are essentially the SAME trans.
Ford updated the E4OD later in production and changed the name to 4R100.
They are still basically the same trans.
The major difference being the 4R100 has beefier internal parts.
I'd still change the rear gears to 3.25, even with the wide ratio trans gear set.