Just replaced my 4.7 engine in my 2000 dodge Dakota with a 2001 4.7 engine. Engine starts but won't stay running without squirting gas into the engine. Any suggestions? Left the original ECM in place...could that be the issue?

This does not sound like a computer problem. Sounds like a fuel issue since the engine will run. Turn the key to the on position and see if you can hear the fuel pump prime up, run until it primes up. There could be a bad wire, connection, bad fuel pump, plugged up fuel filter, or leaking fuel injectors, or the fuel regulator. Fuel pressure should be checked. Both engines are a 16 tooth ring tone reluctor on the crankshaft for the crankshaft positioning sensor, and the engine does run. Check for a fuel issue.