Originally Posted by 66GMC
351M / 400 engines got a bad rap. They were introduced to replace the aging FE series during an emissions transition phase ... so the engineers were scrambling to come up with a suitable powerplant for the light trucks.
The 300 6 was a fairly dependable and even torquey little engine ... but a 6 is still a 6, right?
The 429 or 460 (385 series) is another possibilty ... one with a lot of hot-rod potential.
As a Ford man, the 351M/400 is my least favorite engine because I had them in my shop back in the day and they had really crappy engine management systems. I have seen 400s built to run but to me it's not worth the hassle, and they were mostly gas hogs too. I like my 300 but it isn't the end all of engines. first off there is nothing like the sound of a V8, sorry six banger guys, they just sound like puke for the most part.
I like the advice of going to a 351W, bell housing, clutch and all will still work (not the flywheel even though it will bolt on). Also remember that mixing and matching Ford bits can be (and usually is) an exercise in futility at some point unless you have a complete donor vehicle, or two.
Also on edit, you will need different engine towers to swap. The 300 ones work for the FE engines by just moving them back. Small block use their own and the 351M/400 use their own.