Lax emissions standards had the engine choices behind several years than the U.S. market. For a few years, not sure which specific ones, the GMT400's that had gone Vortec in the U.S. (1996-up of course), were still TBI in Mexico.
You may already know that U.S. production of the GMT400 (GMT455) 3500HD ended with the 2002 model year, but they continued to make them for the Mexico market through at least 2005. Engine choices were the 6.5td or the 8.1 gas, the latter appearing first in the 2001 model.. which coincidentally, was also when production moved from Janesville WI so Silao MX.
But none of that is GMT800 info, so I digress..
Richard