[Rev]

I use a 625 CFM Road Demon on my 306. It seems just about right. I would think that the 670 CFM Holley would be right at the upper limit for your 289. You can get away with a little over size since it has vacuum actuated secondaries. I certainly wouldn't go any bigger on that 289. Set the line pressure to about 7.5-8 psi. That should be plenty and won't over power the needles & seats on the carb.

Rev