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We know that the rear leaf spring Datum Line is usually higher at the rear of leaf spring in a car. How does one determine the recommended height to mount the rear eyelet? Is there a certain percentage we are looking for?

Reason I am bringing this up. I am in the process of replacing the rear frame rails in an old '62 Falcon. I currently have the front leaf eyelet mounted (so to speak, for the third time). The pinon had the leaf mounts welded at the usual mustang angle of 3*. Would I be correct thinking, I should focus on the differential pinion angle, instead of the rear mounting height?
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That angle the leaf spring is at (as seen from the side) effectively steers the vehicle from the rear when it leans in a corner, the same direction the front wheels are steering only less so, causing something called roll understeer which enhances stability at speed. (If it were opposite, the rear end might wind up whipping back-and-forth down the highway in a condition similar to a skateboard with the wobbles.) Kinda helps with packaging also, keeping the floor low under the rear seat. As far as how much angle exactly; "some" is about right, you can measure any leaf-spring-equipped car or light truck built from the sixties-on and copy what they used. Once doing that, then just aim the diff at the trans output (at ride height of-course, and knowing that the engine crank centerline and the rear end pinion centerline need to be parallel but not exactly lined up so the u-joint roller bearings roll around a little and spread wear so it's not just in one spot). On another note you did good by having the leafs spread as wide apart as practical.
 

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A lot of things are going to change as you add weight to the car.

Just tighten your u bolts down so they hold the spring pads tight.

One of the last things you do after you add all the fluids, after firing the engine. Even after a short trip around the yard is finalize the pinion angle by welding the spring pads.

Set your pinion angle close for now so you can do clearence test (jumping on the rear bumper etc).

On the spring sliders you need to guess a bit on your final weight and how your suspension is going to react. Generally if you set the bolt 1/4 the way through travel with just the frame then once you get the weight on(at rest) the bolt will end up around 1/2 way in travel and when you hit the throttle the bolt will slide back to 3/4 or 7/8 the the way back.
You can check unloaded travel by lifting the frame letting the axle/tires(or simulated tires) pull the leafs down to make sure your not coming off the front of the slider. 1/4 at rest is usually just about right for just a frame as a general rule.

Just tighten down your U bolts and tack/bolt your sliders for now getting your pinion close.
 

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Also at this stage if you plan on running a traction aid like assassins or caltracs make sure you leave yourself enough ground clearence under the front of the leaf to keep the setup above the tires sidewall.
 
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