Here is a interchange guide.
http://www.wallaceracing.com/interchange.htm
other good sites
http://www.pontiacpower.com/index.html ( good place to send heads for porting...)
http://www.jbp-pontiac.com/index.html ( good place to buy gaskets and the like)
Ebay is about the best place I know to find pontiac heads... lots on there.....( unmolested is best)
http://www.wallaceracing.com/ultimatehead.htm
/\ basic flow #s ( take them with a grain of salt)
If you looking to make big power the Ebrock heads are the only way to go.( ram air IV exhasut)
If you plan on using a pair of cast iron heads--be careful on who does the porting lots of ppl that don't know what they are doing makeing heavy paper wiehgts.