Long winded response, my apologies in advance. But bare with me, I've been there. I know what I'm talking about.
In regards to timing: Your ignition timing can be dead nuts on target, but if the original nylon coated camshaft timing chain is excessively worn (likely for something that old) and jumped a tooth or 2, it'll never fire up. You need to pull the valve covers, pull all the plugs, use a dial gauge on the rocker arms and SLOWLY hand crank the motor while watching the valve to determine if this is indeed the issue. If the #1 intake valve opens up long AFTER it passes top dead center, you have a big problem.
As a gearhead grease monkey with several decades of experience under my belt, I have seen this scenario countless times with old vehicles. Not an easy fix either. I feel for you.
The very FIRST thing I'd do if I were you is test to see if you're even getting spark. The easiest thing is to give it a shot of carb cleaner, NON CHLORINATED Brake Klean, or even a splash of gasoline down the carb, then crank it to determine if it has spark. If it kicks and fires up momentarily, then yes, you have spark. The ignition timing can be off some and still kick the motor over for a bit if the rest is okay.
Then it needs GOOD gas and the means of delivering it. If it's been sitting for years, it's a good bet the carb jets are varnished over. It's a given it needs tearing down, soaking in solvent and rebuilt with fresh gaskets and seals.
Previous commenters mentioned a Magnum V8 starter. I also have a 73 Newport with the 400 and that's what I did. I didn't use the electrical connection adaptor, I made my own. But I agree, they are better than the big original starters designed way back when. Magnum starters are smaller, lighter, have MORE torque and use less voltage, it is superior. And you can buy a new or rebuilt starter and it be internally shorted and rapidly drain the battery right out of the box.
Get it to crank over without excessively draining the battery first. Take that monolith starter back and trade it for a Magnum V8 starter, they work GREAT! Maybe fresh battery cables too, only thing it'll hurt is your wallet. Then test for spark. Get some carb clean or brake clean while you trade in the starter. Be prepared to drop more coin for a carb rebuild kit. I personally wouldn't buy a rebuilt carb, my experience is half of them are bad out of the box, get a kit and do it yourself. Only then will you be ready to determine of the timing chain is bad. Everything else can be dead nuts right and proper, but if it has jumped the timing chain more than one tooth, it won't run proper, if at ALL.
Last of all, know you have my heart and interests in your favor. Welcome to the club of Voodoo Rocket Surgery, resurecting the dead and making them fly! All the cursing and swearing is what qualifies it as black magic!