The 215 guys like them because they have bigger ports and valves than the 215 heads. They will bolt up to a 340, but the ports wont line up perfectly. The 300/340 cast iron ports are 1/4" bigger in both directions and the valves are both 1/8" bigger. They can be ported to match the 340 intake ports and bigger valves can be installed with machining for larger seats. So are they desirable? I have no trouble getting $500 for a nice set, so, they are $500 desirable.
I thought they were relatively rare, and maybe desirable, but I guess not. http://v8buick.com/index.php?threads/1964-300-aluminum-heads.362178/
Going price for a rebuildable set has been right around $300 for awhile now. They aren't exactly rare, but not exactly common either. I have a set on my 340, unmodified, but I'm also running a blower. Still, with 5 psi of boost I'm happy with them. The weight advantage isn't as much as you think, the iron heads are pretty light. The TA heads are not much lighter than the iron ones. The important difference is the chamber size. These heads have a considerably larger chamber than any of the other BOPR heads except the iron factory heads. They also have the largest ports and valves of any BOPR heads except iron and TA, and possibly some other high dollar aftermarket heads. Jim
