How do I safely get a 66 Ninety-Eight running after sitting for 20+ years?

First, drain the fuel tank completly. You definitely don't want to just pump fresh gas into a tank that's held old fuel for thirty years, beacuse the residue and degradation will gum up your lines and potentially the fuel pump. Once that's clear, follow up by flushing the entire fuel rail and all the lines leading up to the carburetor. Use solvent or compressed air to clear any varnish or gunk that has settled in the system. After that, you can then focus on the engine itself. Do not just drop fresh gas in and hope for the best. The engine needs a thorough initial check before turning it over. Before you fire it up, you need to pull the spark plugs and inspect the condition of the plugs and the wires. Check for any corrosion or signs of damage that might be hiding in the spark plug wells. If the engine seems completely dry and the internals look fine, then the fogging process is the right call. You want a controlled burst of fuel and compressed air into each cylinder to lubricate and start the combustion process without stressing the system with a direct, full-throttle start on a compromised engine. This allows you to gauge compression without risking internal damage from dry starting. As for the carburetor, it's almost certainly seized or heavily varnished. While simple jets might be accessible to clean up yourself, a complete rebuild or a proffesional cleaning is ussually safer, expecially for an engine that hasn't moved in decades. Getting a professional to clean and adjust that unit is highly recommended before you even consider running it, because if the metering is off, you're just going to flood the cylinders and waste the effort you put into the initial priming.

The order of operations should be: fuel system flush, spark plug inspection, carburetor service, and then the controlled fogging and start attempt. Stick to those steps and you'll minimize the risk of making a problem worse.

Before you even think about cranking it, drop the oil and filter and pull the distributor cap. I revived a 64 Starfire that sat ten years and the cap was full of a mouse nest, wires chewed to bits. Mice love a garage-kept car, check under the hood and inside the seats for nests before anything else.

And mind the brakes on that thing. Old fluid pulls water over the years and the wheel cylinders will be seized or weeping. Don't drive it anywhere til you've bled them and looked for leaks. Old drum brakes plus a heavy car like a Ninety-Eight is no place to find out the hard way. Also crank it by hand with a breaker bar on the crank pulley bolt first. If it turns over free, you're in business.

Before anything with a battery in it, pull the plugs and turn the engine over by hand with a breaker bar on the crank pulley bolt. If it turns smooth, you're in business. If it stops dead, you just saved yourself a starter and a lot of grief. While you're under it, check the coolant. Old coolant turns acidic and those 60s blocks don't love rust in the water jackets. I'd also plan on all new hoses and the fan belt regardless, rubber from back then is basicly chalk now. The carb's honestly not scary if you take pictures as you go and lay parts out in order, but the kit is cheap enough that I get why people farm it out.

Update: took the guy's advice about critters and popped the hood first thing. Found an old nest behind the air cleaner, thankfully long abandoned, but a couple wires look chewed. Also tried the breaker bar trick on the crank pulley and it turned over smooth by hand, so that's a relief. Do I need to replace those chewed wires before even attempting a start, or can I test the rest first?