The 8.1L (L18) is a big block. It is not the same block as the LS1 - not even near the "same mold".
The big block is available in the Chevrolet Avalanche, 2500 HD and 3500 GMC Sierra and Chevrolet Silverado, Chevrolet Suburban and GMC Yukon XL all as an option. (I believe) Along with some commercial applications also.