Fighting American is owned by Joe Simon and/or his estate. He licensed that and Boy's Ranch to Marvel to reprint. If you go to his website
http://www.simoncomics.com , he has copyright notices up. given that he's fought for many decades with Marvel over what rights he did or did not sell back in the 40's, I'd trust any copyright claims to have been verified by lawyers.
Simon and Kirby had a publishing agreement with Crestwood/Prize. They owned some of the titles, and later sold some of the titles to Crestwood/Prize when Crestwood staked them for their own publishing imprint. the Black Magic reprints from the early 70's reprinted Prize published material, but was licensed from Simon (and DC basically bought Prize out in the 50's for the Young Love/Young Romance books). So basically any Crestwood/Prize book that is Simon/Kirby related is probably still copyrighted (unfortunately). I've always wanted to try the Bullseye stories, but Simon has never licensed them for reprint. But any non-Simon/Kirby Crestwood/Prize material is most likely out of copyright.
as for Sad Sack, there was a long legal battle between the Harvey estate and the creator (George Baker?) over what rights he had sold way back when. I never heard what the final results were over that dispute, but the very fact that it's being disputed means that someone renewed the copyrights somewhere. So I'd avoid those books too, unfortunately. I remember really enjoying some of those books when I was younger.