The 400 came with a C-6, not the C-4 which has the same pattern as our ZF bellhousing. SOME 400 blocks has cast in bosses for both bellhousing patterns. Keeping in mind you need to drill the smaller C-4/ZF pattern VERY precisely, this would make the swap pretty easy. Otherwise, I believe the 400 C-6 pattern would be the same as the pattern found on the 460 big blocks, which are now being swapped and used with the ZF. I'd call Steve Wilkinson for advice on this adaptation. 562-634-3434, California. Likewise, motor mount bosses may need some addressing, but this is not as critical as getting a bellhousing centered.
Just my 2ยข
Larry
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