How hard would it be and what else would I need to put a 4.3 in a 2001 besides the engine and ecu

 

That would depend. Do you have access to a machine shop, plasma cutter and a mig welder? And do you have the fabrication skills to use the before mentioned equipment? If so it is not a hard project. If not then you going to need a wheel barrel full of money to have parts made. Stock manifold will not clear custom headers alone would close to $1000.

3800 is a better engine and can be bought cheaper then a 4.3 anyway.

 

You'll also have to replace the transmission, as the 4.3 has the same bellhousing pattern as a V8. The 4.3 won't bolt to your trans.