A spring perch does not care what kind of spring is sitting on top of it.
A spring perch will not affect the length of the spring or the length of the shocks.
Maybe you are talking about shock mounts:
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http://www.4crawler.com/4x4/4R_suspe...shtml#R-Shocks
And for the shocks, best approach is to measure the compressed and extended length for the shocks you need then pick something that matches those requirements:
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http://www.4crawler.com/4x4/Rancho.shtml