To answer your initial question...

Quote
Can I cruise 65 w/3.90 rear end?
Sure you can. With overdrive!

But changing to a 3.38 or numerically lower would work too. I have a 3.73 rear end in my '66, but the overdrive in fourth gear puts me at 2200 rpms at 70mph. To me that's cruising.

-ftyler