I have never dealt with them but everybody seems to talk very highly of Randy's Ring and Pinion. Good prices I hear too.
As far as your gear ratio alot depends on how tall your tire is. If your running 3000 at 65 mph it sounds like 3:55 gears (figuring a 27" tall tire) but I never heard of anything newer than 71 coming with gears that low. Most of them came with very tall gears like 2.76's or the like.
If your going with a 4 speed auto overdrive you can run even lower gears for better acceleration. 4:11's would be great. Smokin takeoff and when it shifts into OD your crusin at a decent rpm.
Have you done any measurements to see if the OD unit will clear the trans. tunnel yet. I haven't done one but read on other boards that the OD unit is so big it hits the tunnel so you have to enlarge it some.