Yeah, that's odd, I thought the 400 rear main was different too.
I tend to be leary of 'fit all' parts (all thread and plumbers tape, sure, but a lip seal?).
If a 400 has never been align honed, it can use the same rear seal as any SBC. It if needs to be align honed, the seal lip in the block and main cap have to be machined to a larger ID,to allow the honing mandrel to pass thru the rear cap and hone the housing bores.Felpro and others makes a special rear seal for 400's that have been align honed, and at that point they can't be swapped(align honed/not align honed) or it will leak.