Im just curious, Ive been told that I would have to weld a bigger yoke onto the driveshaft that connects to my 8 inch rear in order to put in a 9 inch rear. Does anybody know if this is true, or would i have to change my driveshaft at all to go from 8 to 9 inch. Also, the axle shafts are interchangable right? Just trying to get some final things down before i go ahead with the swap. Thanks in advance.

Funny you should mention this. I just finished putting together a 9-inch for my mustang. Mine is a '73 Fastback with an 8-inch rear. I found a '73 MachI with a 9-inch for a donor. Everything looked good, and yes the u-joint in the end of my driveshaft from the 8-inch fit perfectly into the 9-inch. I may have just lucked out that both yokes were for the same size u-joint. I do know 'cause I saw it in a Ford Motorsport catalog that they make a u-joint with two different size caps to fit a driveshaft with smaller u-joints into a rear with a yoke set up for a larger u-joints. But what I did find out is that I need to shorten my driveshaft to accomodate the 9-inch rear. Don't know about the axles though, the 9-inch I bought came with axles.

Check with your local NAPA store. I use a NAPA 372 U-joint to connect Chevy drivelines to Ford 8 inch rears all the time. The two cross caps are different sizes to match the driveline from a Chevy to ford rear diff yoke. I suspect that NAPA must also have different U-Joints for other cross applications.

You can usually get u-joints with different size cups to make a mis-match driveshaft / rearend compatible, but you might have to go to a driveline shop to get it. NAPA might have it also. You need to check the axle bearing size in your rearend, they made a couple of different sizes. I wouldn't want to count on an 8" axle sliding right in to a 9" housing because of this.

Thanks for the help guys. My car is, you guessed it, a 66 mustang. The 9 inch rearend i found is out of a 66 mustang also, so its the early style small bearing kind, does this mean that it'll use the same bearings and reataining plates (i think thats what their called). I am currently driving the car around still so i cant really take it apart just to check. Also does anybody know it they will take the same axle shafts, they are both 28 spline and the same housing width. I guess ill just take the driveshaft to a driveline place to have it shortened/re u-jointed.

You might luck out here. Most of the bigger bearing housings were found in the heavier cars like station wagons, Torino's, etc. And I think most of the bigger bearings were 31 spline. That doesn't mean the high performance Fords didn't get the bigger bearings though. The only safe way is to measure the diameter of axle housing at the bearing end, pull it apart or consult with a parts guy who could tell you by the year and make of the cars they came out of.