I have a 9 1/2 inch Dynamic convertor behind my 408 SB and so far Iam very happy with it.
My car weights 3400 lbs with a 3.73 gear, my convertor flashs to 4800RPMs and I even put 3.23s if I drive it out of town.
I would like to try an 8 inch but Iam afraid driving up the mountains to get out of the valley with 3.23s the 8 inch convertor would not be happy.