On my 1970 240Z, I've installed an R200 and half shafts from a 83 ZX. I concur about the lengths being the same on the half shafts. During my installation, without springs on the rear struts, I discovered that in fact there was a bit of binding on the left side when the half shaft was perpendicular to the diff. The passenger side was good to go. So, I shortened the half shaft for the drivers side by 0.500".....problem gone. The machine work to the half shaft is very simple. After disassembling the half shaft, (be sure not to loose any of the ball spacers), replicate the existing male splined end of the shaft, a 1/2" shorter. The only issue I ran into was finding a lathe that has a big enough thoat to allow the U-joint passing through it so the shaft can be chucked up!
Tom