The 400/351m uses the same bell pattern as the 429/460. A 390 (FE series) will not fit, nor will any other series bell.

A C6 from one of those engines will work. For a OD trans, you are stuck with the E4OD (requires a stand-alone controller, $500) unless you buy an expensive adaptor for an AOD or Chev trans. ($800+/-) And those transmissions need to be beefed to take the power and weight of your combination.

You trying to get better mpg or just reduce rpm on the highway? And what vehicle, gears & tires?