I think the biggest problem with these heads you have already addressed.They had small valves(especially if they were on a 351) and the huge combustion chamber.They can probably be made to breathe as well as any small block windsor factory head.If you are just building the engine maybe you can get some pistons that will bring your compression up with these heads.I don't know how much you can safely mill them,around .040 I think but I don't know what kind of CCs you'll end up with.If you mill them this much you will have to have the intake milled too I believe.I'm sure you'll get some more definete reply's.