Check what # heads you have the 1974 and Later 350s had 882 or 624 heads . The the 882 heads if not cracked can be ported to make more HP . The 624 heads are the thinest cast 350 heads ever made best to change them . Now the 3970010 Block was used from 1969-80 you may have a good set of heads on there . The best 76cc GM heads are the 487 or 993 castings they are 76cc with 1.94in 1.50ex valves and a thicker casting .