I suppose the operative word here is "Nearly".

Almost every head ID site you go to shows that the YT 330hp 400's came with EITHER 16 or 62 heads. And, most sites seem to indicate that some other '68 Bird engines may also have come with both head numbers.

I suppose it might be possible that they ran out of 16 heads before the '68 model year was over, and began using the 62 heads. I have no idea if this is true or not. Some sites show that the 16 heads were used on some '69 engines. Other sites seem to indicate that the only 16 heads used after '68 were the small valve version, and the SD455 heads. Again, I don't have a clue as to what the actual truth is. ???

http://www.wallaceracing.com/enginesearch4.htm

http://ronspontiacpage.com/reference-pages/stockheadspecs.htm

http://www.hemmings.com/magazine/mus/2009/12/1967--68-Pontiac-Firebird-400/2534411.html

http://pontiachunters.com/pontiac_cylinder_heads.shtml

http://www.bassettracing.cz28.com/custom3.html

I think it is common knowledge that most every Pontiac engine ID site has mistakes. It has also been proven that there were lots of things done on the Pontiac assembly lines that were not supposed to have been done. I figure that the wrong parts were used many times, when the correct parts were not available, in order to finish building the car.