im gettin ready to rebuild a 350 i have a 1971 block, and i have 2 sets of heads, 1970 #6 heads, and 1971 #7 heads, which heads are the better choice? and why? thanks guys, btw, this motor is going to be mildly built using performer rpm accesories (intake and cam)

 

Functionally the #6 and #7 heads are equivalent. The two differences between them are 1) the #7s used valve rotators on the exhausts only (#6 used them on all valves) so the exhaust spring pockets are deeper than the intake pockets. A good machine shop will shim the springs as appropriate so this isn't an issue. 2) the #7 heads have factory induction hardened valve seats, however if you plan to install larger valves or even if you cut the seats excessively for a valve job, you can easily remove this thin factory hardened area (making them the same as non-hardened #6s).

Bottom line is that there's not enough difference to matter.