According to my GMC Master Parts Book, there are 2 different heads for the 336. Prior to engine s/n 33635001 it used head #530377. Effective s/n 3365001 it used #533777. Also the heads from a 288, 316 and 347 are different part numbers.
How to determine what the difference is would be anybody's guess. The book says "Includes plugs & studs" so I bet these part numbers won't match the casting numbers on your heads. It may be the castings are the same for both 336 heads, maybe not.
As far as using Pontiac heads, I'd bet they would work but can't say for sure. Hopefully there's a Pontiac man on board who can help you further.
