Unless you have an overdrive transmission you do not want 4.11s (even then they're a bit much). They're too much gear for street driving other than Saturday nights. Had some so I'm talking from experience. You won't be driving anywhere over 55 MPH!!! I had 3.50s last year and dropped back to 3.25s this year. Running 3000 RPM at 70 MPH with the 3.25s.
For a 289 an 8" will be fine. Everyone will tell you to get a 9" but be realistic, the 289 (I have a modified one myself in my '66) doesn't make big block power so why spend the bucks to make the change.
Is a Posi (ok, before someone complains again, I know Ford calls it a Trac-Loc but think kleenex, xerox machine, scotch tape....) anyhow, is a posi really needed? You'll pay $$$ for it. I have 3.00, 3.25, and 3.50 pumkins for my 8". The 3.50 is the only posi I have. Don't worry about a posi unless you're drag racing and then it's not a daily driver anymore....
Where to find one? Junk yard! 60s-80s. Mustangs, Mavericks, Pintos, (yes Pintos), etc.
Count the teeth. Example: 39 ring teeth divided by 10 pinion teeth = 3.55 gears! | 
