I researched and found that there are 4 types of Aluminum LT1 Castings. They are:
10128374
10205245
10207643
12551561

What are the differences between them? And which ones are good for porting??? In other words, how do they rate from 1 to 4??????

 

I know the 561 cast heads are the heads on the 96-97 LT1 camaros. They flow roughly 10% more CFM than the 643 heads that come from, what I believe is the 93-95 LT1's. Since they flow more due to larger ports, there is less meat on the head to allow porting. So if you want the best factory head, the 561 cast (which flow almost as good as the lt4 heads) are your best bet. But for people who want a good port job done, the 643 heads are your best bet. Hope this helps some.