I see problem here, a 3.90 gear should give you about 2700 RPMs at 60 MPH not 3700 RPMs. If you are turning 3700 RPMs at 60 Mph then your transmission is not shifting into high gear, check the Modulator and hose to it .
A 3.22 rear gear would give you 2350 RPMs at 60 MPH. 

The ’57-58 car 9” rear ends are pretty much a direct bolt in for the ’55-56 Ford cars.  As mentioned, a different u-joint is required.  The ’59 housings are the same widths but the spring perches are slightly more outboard.  I have seen the '59 rears used with the springs being slightly arched outwards in the earlier Fords but I’ve gone to the trouble to simply relocate the spring perches on those slightly inboard in which to compensate.
 
The rear end housing in my ’55 Customline is out of a ’58 Fairlane and I used it in its entirety with the original ’55 emergency brake cables hooking up as they should.  I did have to modify the stock drive shaft to use the later model u-joint.

Lorena, Texas (South of Waco)

RB        Thankyou for the photos and measurements of your rear axle. It looks exactly what I have. If the ring and pinion gears are good and runable, I am interested in it. Also if you can confirm it is a 3.22 ratio. What is a good time and number I can contact you? Thankyou.   

Eric 

email me at brechlrl@charter.net