66 Dart GT HT Whatsitgonnabe?

I would not worry about the 741 rear... Those are very strong, especially good in a light car like yours. I have a 741 in my 74 Satellite 340 with 3.91's and it's been a good rear, my friend has a 741 in his 70 Coronet with a 440 and no issues there either...only thing is I think there is a smaller selection of available gear ratios for the 741 aftermarket vs the 742 or 489.