Read something today that points out all the differences in the gen. III's. Some of what I read was a little vague.
So, if the 6.0l's have a thicker flange at the rear seal area of the crank, do they also have a thicker seal ?? More importantly will the rear main cover, that houses the seal, be different ?? Found a good ls1 cover, and was thinking of picking it up for the 402 project. lq9 block being used and did not want to buy wrong part.

 

All the same covers

 

Only early model 6.0s have a thicker crank flange,that was because originally the were available only with a 4L80E transmission.Later 6.0s used a narrower crank flange to accomidate the 4L65E tranny used in Escalades and the SS.Early 6.0s also have cast iron heads,instead of the later model 317 heads that incorperate porting simaliar to LS6 heads.Either the 01 or later LQ4 or LQ9 would have these better parts for your 408 project.