The 351W heads were the good ones in those years. They were an improvement over the 289/302 heads. I bet the later model GT40 heads were an improvement over those.
351 W heads had bigger ports and valves. Your biggest problem with 71 heads is the larger chambers, mill them, and the stupid smog port on the roof of the exhaust port, port it out. The GT40 heads are supposed to flow the same as 351W heads. We used to put 351 Windsor heads on 289's with SBC Stainless valves or SBF stainless valves if using rail rockers and BBM valve springs. Worked real nice! I'd use what you have.