My 340 car has a 7/8" bar. I believe at least some 383 A-Bodies got the 15/16" bar.
Edit: I think I was wrong. The parts manual for '68 & '69, and '70-'71 are only showing one bar each year. 2535533 is for 1968, and 2535534 is for '69-'71. I don't know the diameter of the later bar, but for the four years listed there's only one A-Body sway bar each year.
I'm pretty sure I got it wrong because Energy Suspension shows bushings for both sizes as applying to 1968 Barracudas.
