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I have an 82+/- Ford truck Dana that is going to be cut down and built for a 68 A12 spec Road Runner. I believe it has 35 spline axles and I have been told I should change the side gears to 30 spline. Not a big deal since I will buy axles and go through the unit, but my question is why?

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If it has a good power-lock or track-lock I would use whatever spline the side gears are.If they are 35 spline don't change it to thirty spline unless you need it for the sure-grip you are planning to use.
Doctordiff can set you up with a good set of axles and a good sure-grip
Gus