the crossover should have the same pressure as the rest of the cooling system, 12-18lbs.
you could test the crossover by bolting plates to the ends, then using a chunk of bicycle tube with the valve stem, clamp one end to the inlet of the crossover, with the other end of the tube chunk clamped to a solid piece of plastic, metal, wood, whatever.
use lots of clamps on the tube chunk so that the tube doesn't just blow up because it isn't contained. [that sounds way more complicated than i described]
then fill the tube with 20-25lbs of air and check for leaks.
i have used this method for over 40 years, and it works great !