I think it is a head where the intake and exaust are on oposite sides of the head. I understand that the valves were small and made this one an undesirable head. Someone actually using one could tell you more.

 

Ksimmons description is correct. There is no readily available crossflow head for the L6 motor. There was a factory head and a aftermarket head from a company in Japan (about $10,000). The head that Ksimmons was refering to is a NAP-Z head but this is for a 4 cylinder motor, and it does have small ports and bad runner configuration. This head was for better emmisions.

 

LY head (factory) OS GIKEN (aftermarket) S20 (432 & Skyline applications) RB (Skyline and others). Many manufacturers use this design. It does not affect valve size, but does allow for the intake and exhaust to be seperate and distinct. There are advantages and disadvantages to both depending on what you are looking for,ie. power,emissions,clearances,etc.

 

We're all missin' the obvious...

Crossflow does not refer to valve placement, but the fact that the Intake manifold is across the other side of the head from the Exhaust manifold. On a crossflow in-line engine head, the intake would be on the right and the exhaust the left, looking from the front of the car, for instance.This differs from our L-Engine's non-crossflow head with both the intake and exhaust manifolds on the same side of the head.
The valves could be splayed in any one of a number of configurations with a crossflow setup, depending on combustion chamber shape/design. As H4 Stated, the LY head, though crossflow, had valve placement very similar to the standard non-crossflow head (both intake and exhaust on the same side of the head). The TC24B1 head uses four valves, centrally arrayed around a Hemispherical combustion chamber giving a "true" cross-flow in the chamber.
In years past most economy in-line engines and industrial engines used non-crossflow, while performance applications used crossflow. That is now almost universally changing to universal crossflow usage for better specific output.
More than you wanted to know, eh?

 

has anyone seen or heard of one of those OS heads in action on a street car?

 

All "V" engines, i.e. V-8, V-6, have cross flow heads. The intake is on one side and the exhaust is on the other.

 

Its the head part of your car that crosses over when it flows. Very rare thing and something you should really get.