On my Fairmont, the first set of headers that we made, had number 3 and 7s primary tubes crossover under the oil pan, and dump into the opposite collector, purely for packaging . The front 2 tubes on each side hung down, similar to the Hooker "race" FE headers, and the 4 & 8 tubes were considerably shorter than the rest. After those headers basically fell apart after 14 years, TomP built me a new set of 2" primary custom headers, with no cross over, that were much closer to "equal length", and tucked up tighter to the engine, so no more crunching the headers coming down from a wheelstand. And the car ran exactly the same with both sets of headers. Equal length may be a good thing, but if the tube is a different length, but a smoother, less restrictive bend, I think that is more important.