A998 Trans in '66 Dart

I think that the gear ratios might be the torque handling difference.
So, to me ,I'm guessing the torque handling difference was the fall-out of the gearchange,not the other way around.

That 2.20 rear gear of the Chryco's needs the 2.74 low in the 999, for a starter gear of 6.03
But 2.45gears can use either ratio depending on 225 or 318, so starters of .....6.00 and 6.71
And by 2.76 rear gear, the 904 is fine, with a starter of .............................................6.76.
Thus it is possible to have similar starter gears but vastly different cruiser gears, namely;
2.76,2.45,and 2.20. Kindof a poor-man's overdrive, when performance is not required.

Long ago, I pulled the 2.74 ratio stuff out of a slanty case and stuffed it into a 904 case to get a wide ratio 904 without loc-up. With 3.55s, this makes a starter of 9.73, which is the equivalent of a 904 with 3.97s.
To that,I added; a 2800TC, headers, a TQ on a small-port intake, and 5500 rpm valve springs. This worked great behind an otherwise stock smogger318, in my Barracuda, and was a funtastic combo.
I also ran that combo with gears down to 2.76s, and with the 2800TC, they were all doable.
I had all this stuff laying around for decades,so no extra cost to me.

There is one caveat; you must not mix 904 planetaries onto 998/999 output shafts; or vice versa. The spline count is the same, so they do fit; but the shape of the splines are different, and the 904 shaft will strip the splines out of 998/999 aluminum planearies in just a few shift cycles. and Zing!, yur walking.