is it possible to swap a 4.3 vortec engine and transmission out of a 1998 silverado with a350 engine and transmission from a 1974 C 10

It can be done, but you won't pass any emissions check, so depending where you live it may not be legal to drive on the street anymore.

Rich is right. It would be a pretty large project to adapt that engine, but it can be done. But going with an engine from 1974 to a 1998 vehicle, you will have to delete all of the emissions as the 74 engine will not be compatible. Take the ECU and chuck it because everything will be different as the ECU is programmed to control the 4.3 engine, check it's emissions, timing, air/fuel ratio, etc.. A carburetor from 1974 cannot be electronically controlled. You would basically convert your 1998 to a 1974 emissions and control standard. It is without doubt that the vehicle will not get through an inspection for emissions on a 1998.