Cometic's are a Multi-Layer Steel (MLS) gasket and therefore do not have sealing beads. I do not know if they have a part number for an OEM style 427 block with the reliefs in the cylinder bores. Cometic can make anything on a special order basis, for a cost of course.
Is your block still standard bore? That would be a find these days. A 4.25 bore gasket could be trimmed to match the reliefs with a fine grit cartridge roll on your Dremel or a typical die grinder. Try to keep the gasket clamped together at the area to be trimmed as you do not want grinding material to get between the MLS layers. I had some water jacket sealing issues on a large bore 385 motor with Cometic's that went away with Fel-Pro MLS gaskets. I do not know if Fel-Pro has a 427 FE MLS gasket at this time.
If your are going to modify the gasket you might as well look at the combustion chamber on your heads to see if they are a match for the block or if you have a step there. Next look at the quench pad on your pistons, usually a .250 to .395 wide flat "ring" around the circumference of the piston surrounding the dish or dome, you are looking to see what sharp edges, if any are in the combustion chamber that could lead to pre-ignition detonation that Jay refers to above my post. The entire chamber should be a nice contiguous shape at TDC.
