http://www.mustang50magazine.com/techarticles/4379_gt_40p_heads/index.html
Fletch has a good idea as unless you already have the 255 heads, the Explorer stuff (yes to roller blocks btw) is dirt cheap and one can choose between the GT-40 heads or in later Explorers the GT-40P heads....which are actually better than the earlier GT-40s. Some here already run the P's without a special tube steel header. One must pay attention to the spark plug boot type and/or run the Accel type short length plugs.
http://allfordmustangs.com/forums/5-0l-tech/76769-explorer-gt40-head-q.html
http://forums.corral.net/forums/showthread.php?t=1014187