Barry.  I used your question as the basis for a new post.  For mating a C4 to the Y, I’m using the Performance Automatics bellhousing p/n PA26390 for an FE with an adapter.  In this case, both the rear motor plate and the adapter are the same.  This gets me the SFI certification and keeps my particular combination tech legal.  Here’s a link to an older thread giving more detail on the adapter.

C4 Adapter

And here’s a couple of older pics of the PA bellhousing behind a Y.

Another option is giving Flat-O Products, Inc. a call and inquire as to what it takes to make their Y bellhousing for a C4 automatic kit SFI certified.

Lorena, Texas (South of Waco)