2003 Dodge Ram engine wont fire

There's two things to consider. First, which flex plate did you use, the original one or the one from the Dakota. They should be the same between trucks, but there can be changes between years. Specifically, you have to look at the number and spacing of the notches in the outer ring.

Second, did you remove the crankshaft position sensor before starting the procedure, and did you use a new paper spacer when you reinstalled it? The air gap is critical. If it wasn't removed, it likely is broken. If a new spacer wasn't used, and it was inserted as far as it would go, it also likely is broken. Have you read the diagnostic fault codes to see if a code related to that sensor has been set?

The flex plate issue isn't with the ring gear and teeth. It's with the square notches in the outer ring. Different engines and years use different numbers and spacing of those notches, but that is irrelevant if you used the original flex plate.

It is possible for no fault code to set related to the crank sensor just from cranking the engine. The missing signal is more likely to be detected as a running engine stalls and coasts to a stop. When you have no spark and no fault code related to the cam or crank sensors, you need a scanner to view live data to see if either signal is listed as "No" during cranking.

As an alternative, you can monitor the voltage coming off the automatic shutdown (ASD) relay during cranking. That voltage can be measured at any injector, the ignition coil pack, or either smaller terminal on the back of the alternator. You will see 12 volts there for one second after turning on the ignition switch. What's important is if the signals are showing up from the crankshaft position sensor and the camshaft position sensor, that relay will turn on again during engine rotation, (cranking or running). If it doesn't turn on during cranking, it's a pretty good bet something is wrong with one of those sensors or their circuits.