Never heard of this claim before outside of R models. Have I been living in an alternate universe?
https://bringatrailer.com/listing/1966-ford-mustang-827/?utm_source=Iterable&utm_medium=email&utm_campaign=campaign_2648441
"Going from memory here on the GT 350...full bench breakdown, standard bores honed, stock pistons retained but with different ring gap, more precise head efforts on the valves, springs, and seats. Crank polished and balanced and different cam grind. Probably more but I would have to study data."
https://bringatrailer.com/listing/1966-ford-mustang-827/?utm_source=Iterable&utm_medium=email&utm_campaign=campaign_2648441
"Going from memory here on the GT 350...full bench breakdown, standard bores honed, stock pistons retained but with different ring gap, more precise head efforts on the valves, springs, and seats. Crank polished and balanced and different cam grind. Probably more but I would have to study data."