I was under the impression that R31 springs are different diameter to the bluebird struts? They take the same spring diameter as the conventional datsun struts, right? I thought that R31 struts were bigger on the perch? But could be wrong.
Anyway, start here:
http://wiki.r31skylineclub.com/index.php?title=Springs
I would say no, if not, then marginally stiffer. If it were me, i'd fork out the $$$ and get a set of springs with the rates and ride height I want for my particular purpose. I'd start around the 200lb mark on front and 750lb rears. But thats just me.
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Ongoing Project: 1972 Datsun 510 S13 SR20Det.
http://ozdat.com/forum/viewtopic.php?f=33&t=17898
New Daily: D22 Navara (The new workhorse)
Retired 12/2016: MY98 Subaru Impreza RX
Previous Car: Restored Green 1972 Datsun 510, Hot L18