I have a 9 inch ford w/TKO600 with short 5th (.82 or .84 don't remember) and run 3.25. Engine does 580hp/590tq at crank. Seems to be a good combo. I run about 2500 at 70. I thought about running the long 5th (which is wat I changes from) but did not want the big gap from 4th to 5th. Play with this gear calculator - I found it helpful.
www.secondstrike.com/technical/gearcalc.asp
Bruce