69-70 with 88-newer suspension

[James75]

Hello, this is my first post so I hope I'm doing this right.
I've been reading everyones threads for a while, and I have to say there is a wealth of knowledge in here! I'm starting on a project for my daughter, it's a 70 1/2 ton short wide. On planing this out I've decided to update the drivetrain/ suspension. Plan on using a later 350/700r4 combo but the problem is I'm wanting to use the suspension from the same truck. From what I've read this limits me to an 87 and older. Is there a reason no one is using the 88 and newer suspension components? I've searched all over and can't find any discussions about this. Any help would be greatly appreciated!
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