Will this 71 a833 work in my 68?

You're least likely to run out of gear with the 2.47:1 gearset. It'll wind out more quickly with the 2.66 first. My '69 Valiant is also a 340, but it has a 3.09:1 first gear (1964-early '67 A-body Slant Six close-ratio, sometimes wrongly called the "Commando" gears) with 4.10s and 25.8"-tall tires. It's all done with first gear in about 150 feet, but damn it leaves hard.

First-gear torque multiplication:
2.47 x 3.91 = 9.658
3.09 x 4.10 = 12.669, i.e. how your first gear would feel with 5.13 gears. Weeeee!