I believe the 3975950 heads are from 1968-70 366T, 396, 402, and 427 truck engines. It has 2.06/1.77 valves and closed chamber oval ports. I don't remember if I have never seen this particular casting number, but all credible sources say that it's an oval port, closed chamber head. Check this web site for more details:

http://www.chevcarclubvic.org.au/tec...ng_numbers.htm

Regards,
Glenn