I am getting ready to swap in a 9in rearend. Will there be enough differences in size that I will need a shorter or longer drive shaft? I will be going from the stock 3.92 gears to 3.25 how will this effect perfomance and drivability with a 700-R4 transmission?

Forgot to ask also about lower shock brackets. I have brackets to go on the bottom side of the 3in axle. I am looking for shock brackets that bolt on the u-bolts like what was used on 57 ford trucks.

 

what I hear is if u get the up to 1972 9in rear from a truck its a direct swap.

3.25 performance will be 'slower' off the line than the 3.92, but engine revs will be down as well.

you shouldn't notice much with the 700r4.. 1st gear is nice..

Sam

 

Driveshaft length should not change if all you are doing is changing the rear axle. You might need a conversion joint if the existing U-joint on the driveshaft does not match the axle input.

A 3.25 ratio with a 700R4 will let you cruise on the freeways at a nice low RPM.

 

I don't know about the "newer" trucks but for the '48-52 trucks I was told the drive shaft comes up slightly short by about an inch. What about the width of the axle tubes? Are the newer trucks the same as the later ones? When swapping a 9" into a '48-52 truck you have to either build or buy spring brakets or use the spring brackets from a F-2 because the older 1/2 ton axles' tube diameter is smaller.

Rear Axle Page

I have a 9" for my stock '48 panel, just have to find the time to do the swap.

 

Your 700R4 has gear ratios at 3.06/1.63/1.00/ and 0.70.

With a 3.25 rear end, that might be a little steep.

With 88" roll out tires (235/75R15s) you will be going 65 mph at 2500 rpm in third gear, and freeway speed (65) at 1850 rpm in overdrive.

Even if you go with a smaller back tire like a 245/60R15 you will be going 67 mph at 2000 rpm in overdrive.

An ideal rear end ratio with the larger tires (used to fill up the wheel wells) would be about 3.80.

Here's the rpm calculator for you to play with:

Gear Ratio Calculator

 

I want to mount the shocks in the stock location on the bottom. The axle is going to still be under the springs. The stock mounting plate will not work. Can anyone share what they did and show some pictures?

 

The only reference I have is for the '48-52, I don't know if there is much of a difference, but the best set up is use the mounting plates from a F-2, the F-2 axle is suppose to be about the same diameter as the 9". I also think some of the parts vendors like Mid 50s carries a kit. I'm sure some one will be along soon to offer better information.

 

I just finished installing the 9" rear in the 49. I had heard that the project was a direct bolt in and this confirmed it. I used the upper U bolt plate from the 71 and custom made U bolts. Any spring shop or trailer manufacturer can bend these up for you. I haven't mocked up the motor and trans yet so I haven't installed the shims that will give the pinion shaft the proper stance. I plan on using aftermarket shocks but i will wait until I get the wheels and box back on to get a better measurement.
I hope this helps and good luck with your build.

 

IMHO and experience 3.25 is to tall. I'm running 3.50 and it's still to tall so this winter I'm switching to a 3.73 gear set. I like this gear calculator; MOTOR - A Rear Axle Calculator

 

So I just did mine, Ford 9, purchased the rear-end upgrade kit from CPP and welded on the new shock mounts. I ended up having a new driveshaft built for a 100 bucks. the new rear-end sits under the springs. The kit comes with the new U -Bolts, axle plates and shockmount brackets. I did not use the new axle plates, the ones on the 9 fit perfectly. The new plates are for if you want to put the axle above the springs for lowering.

good luck.