Any advice for replacing exhaust manifold gasket in a 454?

Given the age of your vehicle, every nut, bolt and screw will be rusted tight. Get lots of penetrating oil, a top quality set of drill bits, and stud and bolt extractors. You will undoubtedly run into broken bolts and fasteners when you try to loosen them. Also get a good supply of top grade nuts, bolts, and studs for replacement.