Will I regret change to 3.55s from 2.76s

3.55 might be more than you want.
3.23 is still a lot better than that 2.76 you have!

I am going to run a 3.23 in my 76 Dart Lite.
4 speed OD box.
That is exactly my concern. I was actually hoping the rear was going to be a 3.08 when I bought it as I think that would be a good compromise. Although, I just did what should have been some obvious math. If I drive 2000 miles in a season, getting 14mpg instead of 16, that's only an extra $72 even if gas is back at $4 a gallon.