Here is the story so far...just took a factory Mopar 440 shortblock. Took my old 440, bolted on everything else, new cam/lifters, oil pump etc. Everything looks good. Its bolted in, and spins over nicely. Problem is...it doesn't fire at all. No backfires, no sputtering, nothing. I verified spark at the plugs. Primed carb a little, shot some starting fluid. Nothing. What did I screw up here?? It seems like everything is right. This is frustrating.....

Try using gas instead of starting fluid. It takes a lot to prime that pump. Pour into the fuel bowls and try several times it should fire.

IS IT A AUTO OR 4 SPEED

Its an automatic....are you thinking Neutral safety switch??

I JUST HAD THE SAME PROBLEM TODAY I BACK IT OUT OF THE GARAGE DROVE IT FOR A LITTLE WHILE PARK IT AND IT WOULD NOT START, MOVED THE NEUTRAL WIRE HARNESS BY THE TRANSMISSION AND THEN IT FIRED UP

I don't claim to be an expert on neutral safety switches..in fact...I don't really now squat about them. When you turn the engine over, doesn't the neutral safety preclude the plugs from firing? If mine fire in sequence, that would indicate the ability for the motor to ignite the fuel? So, as a dummy..where does the neutral safety switch prevent the ignition system from functioning? Yeah, off topic, but still a good subject to learn from... and I am out of gas in my spare can cuz the kids put it in the four wheelers today

and for the record...my neutral safety switch was loose...but car still does nothing... I put #1 at TDC, lined up rotor with wire... even if timing was off, it would still pop and back fire out of the carburetor right? I am a veteran at having my timing off

Back to the basics .. you need fuel, spark and compression.

Sounds like you are lacking compression OR your spark is real weak.

Starting fluid SHOULD fire this up ....even with a weak spark.

I used an old Windex bottle with gas in it to spray into the carb while we tried turning it over. I swore we had it lined up right but finally spraying a bunch of fuel in it backfired out the carb. It was 180 out. Swapped it around and it fired right up. If you got spark and you got fire you should have a running engine if the dizzy is lined up right.

How tight was the cam bolt? If it loosened up the valves won't operate. Does the distributor spin when you crank it over?

You sure that dizzy isn't 180 out?

Mine didn't back fire until it had lots of fuel in it. Most of the fuel line was new and the carb had been totally emptied. It took a long time, even after we got it running, for the fuel system to prime and start providing enough fuel on its own. It is great now I'd still be cranking today if it wasn't for the spay bottle fuel of gas.

Check COMPRESSION

David, check your E-mail.

Here is what I would do:
pull #1 spark plug
crank engine till you get cylinder pressure
check damper timming (should be close to tdc)
Make sure dist rotor is pointing at #1 plug wire on cap. (not 180 out)
crank engine so timming marks are about 10 degrees BTDC.
align the dist trigger with the pickup, this should set initial timing close to 10 BTDC.
Put dist cap back on and double check fire order, reminder - Big Blocks are reverse rotation on the dist.
check for spark at plug end (are the spark plug wires good?)
add priming fuel and try to start engine.
If it is really having issues starting/running then pull the drivers side valve cover, crank the engine to TDC of cam overlap and check that both the intake and exhaust are both slightly open to verify cam installation is at least close?
if it checks out good, check compression or do a leakdown test. What was the new cam? could it have caused bent pushrods or valves?

On the Neutral saftey switch, it just allow you to crank the engine, so if the starter works, it is fine.

If the engine will fire when cranking, but dies when releasing the key, the ballast resistor is bad.

Try tapping lightly on the sides of the carb,your floats may be stuck and starting fluid won't help in that case.If your pour gas in the carb and it starts for a moment this may be your problem.

I might try grounding out the neutral safety switch as a test. Just run a jumper from the starter relay prong to somewhere on the body. I had ther same problem on initial fire up of my Challenger and had a brand new safety switch. Threw me for a loop since my switch and wiring was new... Also, the dizzy may be out 180. Been there, too...

air fuel spark if one is mising you get nothing. The N/S switch is a 3 prong plug that is mounted on the drivers side of the rans. Check it. Then do as above, make sure you have it at TDC, mark where #1 is. DO NOT USE the #1 on the cap. Start your firing order there. Prime the carb, a holley is easy fill it up thru the vent, work the throttle until you see it squirt gas. If yo installed a new T-Chain and gears maybe you are off a tooth or 2.
To check for spark pull a plug hold the plug wire and pump the swtich...hahahha no don't do that stick the plug back in the wire and ground it to your engine, then bump it a few times. Dim the lights if you can. I'd check for spark at the coil 1st, pull the coil wire out of the distrib cap just enough to see it, bump it you should see the spark jump into the distrib.

He said it turns over so the neutral switch will have nothing to do with his not firing problem.