I just put a used 5.7 from a 97 Savana 3500 in my 00 Express 1500. The Savana ran great but was rusty, the express had a blown motor.

The engine starts, runs for 2 seconds then dies. No security light on, could it still be vats? Anyone encounter this or have suggestions?

 

measure your fuel pressure

 

My previous car, a '98 Jeep GC, behaved the same when alarm was switched on and you tried to start the engine: The engine turned over and started, but died immediately after. You could try again and again but the engine kept dying until you switched off the alarm with the remote.
Very well possible that the alarm disabled the fuel pump (and not the starter and the ignition).

 

@NL-Toontje EFI deactivation is part of the PASSLOCK anti-theft system designed by GM, and installed standard in every vehicle. The feature does not require an aftermarket alarm system.