The distance between the inside of the 2 legs of the u-bolt must match the diameter of your rear end housing or you will have problems. If you have a factory Ford housing somebody will have the correct bolts.

 

When we stuck the '57 9-inch housing in our '65 I had to tap the u-bolts over the tubes when squeeze the ends with Channellocks to fit the '67+ shock plates...

 

Believe I had to do the same 30 years ago when we put the 57 ranchero rear end in my 66 .

 

Based on your photo it appears the difference is significant. Match them against your shock plates and see just how much they are off.

 

The new and old U-bolt both fit the shock plates well because they are Lakewood traction bars with shock plates incorporated and they are slotted.
At the housing end of the U-bolt, the difference is 1/10 of an inch.
I would agree, that is significant.

 

I also have a 28 spline, small bearing 9in hosing and tried several u-bolts. You might try another set.
-All required some amount of spreading open to fit over the tube and some squeezing to then slide into the shock plate holes. I found using the outside edges of bench vise jaws to spread them worked best.
-I like AMK, but at the time found the Scott Drake concours (their product description, not mine) were a better fit.

 

I cannot find the correct size anywhere. On the retail sites some have no measurements or they have center-to-center measurements. The most common is 2-3/4" wide, but like the set I bought that "fits 65-73 Mustangs", that's at the bottom by the threads and it narrows near the upper part where the axle will get clamped.
The new set is 1/8" too narrow at the top where they should fit over the axle tube.

My axle tubes are 2-7/8" and I feel a 3" u-bolt may leave too much slop. The shock plate holes are elongated so they're not going to tighten up the diameter of the U portion when tightened.
I can't believe I have some oddball rear end.
Here's the thing, the existing set I'm replacing and the new set I bought are both tapered, wider at the thread end. The width there is correct but if I force that over the axle tube, they will spread a LOT.
What am I missing?
Both of these bolts taper the same amount, 1/8" from top to bottom.

 

I found them at CJP by Scott Drake, the black ones with the flattened edges and red locknuts. Perfect fit. Not 2.75 inches or 3 inches but right in between. And they are the perfect length.
It would make things a lot easier if the retailers would put an accurate group of dimensions with the parts they are selling. Not doing so takes up everybody's time and increases returns, no doubt. Frustrates me!