Axle shaft differences between 280ZXt and 300ZXt?

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I'm at a stumbling block. I want to get the CV conversion going in my 240z. I need to know if there is a difference in the splined end (spline count, diameter, E-clip groove, etc) between a 280ZXt (six bolt CV) and a 300ZXt (4 bolt CV). I want to use Ross' 300ZXt billet companion flanges (four bolt) to gain additional shaft length. I could use the 280ZXt companion flanges (six bolt) if needed. I'll have to get custom axles machined since I'm using a NISMO R180 with CV style side flanges. That's why I'm asking. Once this is settled I can feel comfortable about dropping $300 on axles just to get the outer CV joints plus another $150 or so for the custom shafts.