71 up 2.47 to1 and 2.66 to 1

For a streeter,
those two boxes are so close, it's hardly worth thinking about.
2.66-1.92-1.40-1.00/ splits of .722-.729-.714 versus
2.47-1.77-1.34-1.00/ splits of .717-.757-.746.

This is because, Second gear will get you into speeding territory after about 5.5 seconds, and before you hit Third gear.. The only concern for a streeter, is the first gear difference, which is about the same as one rear gear size, ie
2.47 x 3.55=8.77 and
2.66 x 3.23=8.59 an overall difference of just 2%.

The 2.47 box has shorter splits on all but the 1-2 shift, which is worth a bit of rpm on the shifts at 6000. This might be an advantage in some forms of racing; but, On the street, any stock 318 or bigger can use either with either gear.
But as cylinder pressures drop by installing cams with later closing intake valves; this causes a loss of low-rpm torque, which is when the 2.66 will come out the winner..... with 3.55s
I sure as heck wouldn't replace whatever of those I had, with the other.
If I'd swap either, it would be for a Commando,
3.09-1.92-1.40-1.00/ splits of .621-.729-.714
This, to get a starter of about the same as the other two, requires
~8.60/3.09= 2.76; which really slashes the cruising rpm.
But you know; good luck pulling Second gear with gusto, if you have low cylinder pressure, lol....... which is why I have a 360 pumping over 180psi........... so I can run whatever street rear gear ratio that I please. This allows me to put Second Road-gear right where it needs to be, somewhere in the mid to high 6s.
With a 1.92 Second, this calls for a minimum of 3.55s. But,
with a 1.77 Second, it calls for a minimum of 3.84s........ which is why I sold my 2.47 box; cuz at that time, I had no overdrive.

I've tried many Second Roadgears both higher and lower than 6.80, but always come back to it, sooner or later. It's just a really fun gear in town, and I can just leave it in second most of the time, cuz it goes at the rate of 11.8mph per 1000rpm, which is from; 30mph@2500, to 60@5000, and even to 85mph@7200

Happy Hot-Rodding