6L80E should bolt up. It's the next generation of auto transmission and will help keep you in your power curve. The Allison will be a bigger pain in the ass to adapt than the 6L80E since it's designed for the Isuzu diesel engine. I would bet money it's got a totally different bolt pattern on the bell housing than a Gen 3 or Gen 4 engine. Anyone seen an Allison on a gasoline engine?