The ideal setup where the crossover can have a real effect is if the pipes off the header collectors are the same diameter (no reducers, as in 3" pipe hooked up to 3" header collector) and are made to specific length of about 3-4 feet (depending on engine setup) before the crossover..
FWIW Pipemax calculations I've done show for a typical 340 puts the best locations for H pipe or X at 20, 40, 82" after the end of the primaries.
Plus minus an inch or two isn't going to matter. The hamonic calculations only change slightly for different cams and compressions, at least within the street/strip type uses.
Worst placements will be around 31, and 61".
