I have a 2006 silverado 4.3 liter. I have to swap engines and I picked up
a 2004 4.3 liter. The engine I bought was using a fuel injection return
line. The 2006 engine was not using a return line it was blocked off
with a plug. Can I just block it off like the 2006 4.3 with no problems?

 

When I did my 5.3 swap from 1999 to a 2003 engine, I just swapped everything over from the original engine. Intake, exhaust manifold, EGR, etc. I would do that if it were me. That way you are basically putting in exactly what you took out.