Any advice for replacing exhaust manifold gasket in a 454?

your selling it. Get it hot, mix up some furnace cement, or buy some pre mixed, and but it on the leak. Let it dry before starting.

I'm selling it so need the simplest thing that will pass inspection.

Get a hold of Ivan at Pine Hollow Automotive in PA !

(P.S. I agree, the manifolds are likely cracked.)

The best advice I can give you is don't just replace the gasket, replace the exhaust manifolds with headers. If you look very carefully the stock manifolds are probably cracked anyways. The old 454's and 460's run hot anyways and tend to crack and burn spark plug wires also. I would also replace the ign wires with Hi temp 10.4 MM wires like the Taylor Extreme wires while you are at it. Your engine will run a lot cooler and the fan clutch will come on lest often, not to mention increasing your HP between 60 and 80 HP. If you plan to do the job yourself soak the bolts every day for a week with Kroil oil and tap each bolt head each day to set up a capillary action to get the oil into the threads. If the bolts won't come out or break off, I would let a garage do it.

Should it be hot or cold when removing the bolts?

Bandaides. Plenty of bandaides for the knuckle-buster bolts.

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.