I know the OEM 6.4 BGE blocks have been built to 1000+ hp, SC of course. The BGE blocks are the truck 6.4 and all 6.4's after 2017 (I think) and have BGE cast into the rear of the block.
Not sure on the pre-BGE 6.4 block, but I kind of doubt someone could find the limit of the block without boost.