I have seen and measured the differences between the two castings side by side. They beefed up around the end head bolt holes from top to bottom, water jacket is smaller in that area, also all along the top where the rocker stand bolts to, there is more meat and there is also a "bridge" or gusset of aluminum that goes from the rocker pad at the end of the head over to the front and back valve cover rail, past the end head bolts. There is more material in between "rocker pads" where the other head bolt holes go, bridging across there, basically running continuous across the entire head even past the end head bolts, versus having 4 separate pads between head bolts, big difference. The new castings weigh 2 1/2 pounds more than the old castings. I have pictures of the two side by side but can't post them at the moment.
Also those particular heads (I took those pictures, but not to post on the web...) are the old castings that had issues, they were also angle milled over .100 and that took meat away from the already weak corner head bolt area, because they angle milled the intake face as well as the deck. Those heads also had 420/1140# springs and .850'ish lift, ran 8400 RPM for years... were the older castings and were replaced by the newest revised BT HR castings run 3 years ago. The new heads (same port program) and new intake made 168 more HP than before, 1087HP versus 919.
68 Cougar XR7 street and strip car, 487/427, 4-speed Jerico, 10.5W, 3560# race weight, best firstname.lastname@example.org 1.34 60 ft