Engine swap

If it's the same size engine then go ahead just put accessories from the 03 engine on it. Then it should bolt in.

Just to add this one it shows there not threaten not sure what's different but there different part numbers. The 1999 shows to only have the same part number for a 1999 alero.

I posted all the years makes and models that are the same part number only 2003 model year.

The blocks should be the same as long as accessories form 03 motor is used a lot of times with engine numbers it may mean a different cam/valve size head or manifold like exhaust.

The only difference wold be if alero is rwd and I forgot if it is but I don't think it is.

It's up to them I don't like to buy a engine or even if had a engine go thru the trouble of trying to install it and find some how something was different so it didn't work for what ever reason. Even a different sensor or connector no the check engine light is on. One engine has air injection the other one doesn't that would really be bad. Iam not trying to disagree with what your saying it might work fine but there is also a chance it might not.