Your typical truck headers will not fit the passenger car because of the motor mount location. The trucks use a front motor mount attached to the timing cover and the rear motor mount is actually a pair of legs/tabs off the bellhousing. This leaves the sides of the block unobstructed so that the trucks can use the Ram Horns or some other tube headers. Cars use motor mounts in the middle of the block. Car bellhousings do not have the legs/tabs off the bellhousing so you're really limited on your choices for tube headers for 55/56 cars.

Jerry Christensen who advertises in YBM can build you a set and John Mummert has plans in the works for Jardine to build passenger car headers for the Y's. I'm using a set of Red's Headers Passenger Dual Tube Headers on my 56.

Hope this helps

Rono