69 Super Bee. 3" MP inboard kit. Front segment is good. Original spring was 22", SS is 20. Original hanger hole to new hole is 2" further back.

My question is where to drill rear shackle hole. I've read about 56" and 57" leaf springs. Dodge and Plymouth differences. I'm now running P3690454, P3690455 which is hard to measure curled up with no weight on them.

I plan on a pretty low ride height, even if I have to dearch the springs.

How far away from the front hole should my rear shackle tube be for a good shackle angle?

I like the bottom of the "vertical" hangars to be a bit rearward (from 6 0'clock vertical) loaded so the spring does not raise the car which it would as the shackle arcs & goes over center if the shackle was pointing forward but rather the spring "compresses" as it works WO raising the car. first I would locate the springs so the axle/wheels are where they look best in the wheelwells

Ya Rapid, rearward angle for sure. So I centered my wheel in the wheelwell. Checked center of wheel to front leaf spring mounting hole. We have a good solid match at 20".

So basically how far back should the rear hole for the shackle mount be for the proper angle be? Im guessing a tiny bit shorter than a flattened out SS spring.

I'm thinking that you might have to go ahead with an edumacated guess for a start & drill some holes for a mockup to get it assembled/loaded then redrill more from there as needed unfortunately

I would move it three inches forward from the orignal hole to make the rear shackle angle better

I think you need to rethink the 3" dimension.

His front hanger is longer to account for the shorter front segment.

this may be a good place to start. I used them on my dirt track cars.... way to complex and I am to lazy to type it all !! it will get you on the page .... https://landrumspring.com/technical/oval-track-leaf-spring-mounting-point/

You may have a point if the O.P. is going to use the longer rear front hangar to move the front hole back two inches and not slide the rear end forward to improve traction by shortening the wheelbase. I normally trim the front edge of the wheel wells two to three inches to move the rear end forward at least one inch,sometimes two inches or slightly more
I haved had several different Mopar leaf spring kits installed into the rear sub frames of, A, B and E bodys. None of those cars had enough rear shackle angle stock to make me want to keep the rear shackle hole in the same relation to stock when switching to the Mopar brand SS leaf springs with the two inch shorter front segments SS rear springs, hence my recommendation to move the rear hole forward three inches to gain one more inch forward on the upper mount to help increase the rear shackle angle more increasing the rear shackle so the rear of the car will work better under WOT
OP, make a mark on the rear sub frames with the same size hole as the current hole at one, two and three inches forward and look at those as a references to see what the rear shackle angle could be and pick the one you like the most Make your decisions from there

You can see the left rear shackle a little in this pic and get a good idea of the angle when everything is in stock location. As everything is in stock location on my 63. And the springs are not moved in any. Ron

There's too many variables to tell where the optimum angle comes into play without the vehicle's final assembly including the dearching of the springs if you go that route...Drilling holes now will be a guess at best and you'll probably end up making more when it's all said and done...
I'd either install the shackles in the factory frame holes until the final assembly is done and you know what you've got to do...Or{if they don't fit} don't install the shackles, use a wood spacer between the frame and the rear spring eye and let the spring slide on the spacer until you're done assembling the car and have added all the weight..

Here's a diagram from the MP chasis manual...

Since the Ss Springs are based on an A body size. If anyone has an A body. The front mounting hole to rear shackle hole should be all I would need.

Can we assume a Ss spring in an A body will have the perfect shackle angle?

57.26" center to center (front pivot hole to rear frame hole if I am reading my 71 FSM correctly.

Or measure original distance and subtract 2".

No, absolutely not! Install your springs first and then look at the rear shackle angles Mopar made a bunch of different SS drag race springs as well as road handling SS type springs, not all of them will work well with stock front mounts and stock rear shackle hole locations

What about installing a slider back there?