This is incorrect. The 2005 Monte Carlo SS came with the L67 (Gen 3 M90). However, you are correct that mmullings is has his information mixed up though because the only engine offered in the 2005 Monte Carlo SS was the L67. If the 3800 came from a 2005 Impala (or 2005 Monte Carlo LT for that matter), it would be an L36, not an L26. You can tell the difference between the two due to the UIM manifolds (L36 is plastic, L26 is aluminum) and the L26 has an electronic t/b.
Good luck with the problems that you're having. At least the shop is covering the dealership costs.