Different Gear?

Yep! Correct you are on your formulation, my 2.76 in the 7-1/4 goes just a little past 1-1/4 turns. I think 8-1/4s were 2.71 or 2.73 ratio, so you should have the carrier type and pinion location that allows you to go up to the 3.21 gear set, or deeper gears, if you like. I put one of those out 8-1/4s with 3.21 gears out of a 73 340 duster into my first /6 duster and made a set of superstock springs by the specs in mopar chassis book out of the two sets of springs, it was night and day between the 2.76 gears. I recommend the 3.21 and converter if you are going to put A/C back in it.