You don't say what diameter you have now. FYI - A 1970 w/ a 440 uses .92" dia and a '71 w/ a 340 uses .90" dia.
I'd say .96" if you have a big block and .92" if you have a small block, since the largest I can find that the factory used on the 340 was .90" (I'd assume that's an AAR or T/A model)
I have a '73 which I'm planning to upgrade from .86" dia. to .96". My car was a small block, so I'm going one step above the heavy-duty size for the big block based on an article I read in the Feb. '06 issue of Mopar Muscle. They took a '67 Coronet through a front-end rebuild and got it to pull .92g average! Not bad. They didn't list what the baseline was, but I can imagine it wasn't too good. They swapped the .92" dia. bars for .96" and recommended not using the 1" because of harsh ride. I'm guessing that the E-body is lighter than the 67 Coronet, so the ride would be even worse when using the 1" bars.