I think the 400 uses the big block bell housing pattern, while the 351C uses the small block pattern. My 70 GT 351C came with an FMX. I put it in my 65 pickup with a 5.0 HO which I got out of an 87 Lincoln. I am putting an AOD into the Ranchero to gain overdrive for the highway.
What I don't like about the 400 is that the Pistons are about 1/4" down from the top at TDC. Ford did that for Emissions reasons, not for performance, it drastically reduces compression. 400's don't generally get very good gas mileage. This swap would be a step backwards to me. I would stick with the Cleveland.
If you really want more cubes, you can rebuild your current motor and install a stroker crank in it for a reasonable amount of money. If you're motor is not burning oil, I would install your aluminum heads with a mild cam, headers and a dual plane intake and see what you think. Way less money, way better motor, way better gas mileage.
Here is a picture of what I am doing to my Ranchero.
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