I am glad you brought up the rear leaf springs. Here is some data I have on rear Chrysler leaf springs - which are 2-1/2 inch wide. The full size pickups and vans use 3 inch wide leaf springs - so are not mentioned here.
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Above is sorted by overall length.
Below is sorted by front segment.
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The front spring hanger comes in several different lengths (from body mount to leaf spring hole).
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The thing is I don't have those lengths for the different front spring hangers (yet), so not sure what we will need. The above list of hangers (for sale) is from
Spring Hangers | AR Engineering.
You want the center bolt hole (differential) preferably in same location. If not, your propeller shaft length will change and not sure how it would look not centered in wheel well.
You also want rear bushing hole in same location - but this can be moved providing the rear spring hanger position is moved (forward or rearward) the same distance the rear bushing hole is located - but this might be a bit more difficult to do. To not do this will mess up the spring geometry.
I happen to have a used pair of '69 GTX 440 HP rear leaf springs in my garage - with the "Plymouth B" front spring hangers. Those spring hangers are about (measured attached to spring - so might be off) is 2-3/4 inch from front edge to center of bolt.
The front spring hangers I have (measured on car - so might be off) are 2-3/4 inch from front edge to center of bolt - so might be the same as above (...maybe).
The problem I see is none of the above leaf springs have the rear segment length, of 34 inch. I was wondering what you are going to do for this has been a stumbling block I have. To me, it appears I need to have a new top spring made to fit my car, but with a shorter front segment (using a different (longer) front spring hanger), then either use the lower leafs from my 53 year old '69 440HP springs, or get a new set of either SS or 440HP/Hemi springs and merge the two.
For those who don't know, the 440HP/Hemi leaf springs (B-body or E-body variants) have 5-leafs on one side and 4 full plus 2 half leafs on the other side. I "think" the SS springs are different.
I'd like to hear your thought - for maybe you have other insights I've missed.
BudW