The YH engine code may be 428...or it may be 421, 400, 455, or even a 301.

You will need to find the block casting code behind the right cylinder head. It's cast into the upper webbed surface of the block and will typically be caked with oil and dirt.

In addition, there is a block casting date in the vicinity of the distributor base, along with the block casting code it will positively identify your engine.
http://www.yearone.com/updatedsinglepages/id_info/pontiacengineid/pontengid2.html