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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?

Since you are building from scratch you should use 35 spline stuff since it cost little or no more for axles and 35 spline side gears are relatively cheap for a trac lok or power lok if your's doesn't have them. It's good insurance in case you decide to put some drag slicks on it.