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When you say 56" springs, are you measuring from center hole to hole at each end??

The 56" is from one end to the other.
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Here is the other problem, they are all still on the trucks, so they have a load on them.

This shouldn't be that much of a problem, the wrecking yard shopuld either remove them for you(for a price), or lift the rear to allow you to remove the springs yourself.

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Is there another way to get what I need


Yes, you could have a spring shop build you another main leaf, yoiu would take your packs in they would do all of the work. This should run you about $150-200 for the spring and labor. I am just guessing at the price since the last time I had it done was in '94 and it was only $40/spring.

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like what year truck are they common on?

The 56 chevy springs are on a lot of Chevy's, the last set I picked up were on a Suburban. You could also pick up a set of Toyota, Nissan, Mazda leaves if you found a set that was already removed and use the main leaf from the pack, you would need to cut off the ends. I have just alwasy grabbed the Chevy's since they are easier to get to.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed