[cobrajeff]

The C6 that came off a 351C definately will NOT fit your FE. There are two basic C6 configurations "big-block" (which is by far the most common) that fits an FE series engine, and "small-block" (usually found on a 351C) that will fit any of the standard 6-bolt small-block engines (289, 302, 351W, 351C).

The C4 transmission is a "small-block" tranny, and has a separate bellhousing. There are no "factory" C4's that will fit an FE, but, there are custom aftermarket bellhousings available that will allow you to mate a C4 to an FE. the C6 bellhousing is an integral part of the case, so they cannot be "adapted" in that way - you need a small-block C6 to attach to a small-block engine, and a big-block C6 to mate it to a big-block engine.

As the other guys recommended, you need a big-block C6 to mate to your 390 FE engine.

regards,

Jeff