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I asked before and the community decided it was too big too heavy and too wide, from what I've seen and read the 400 is based off of the 351 with the only difference being a taller deck height. In my opinion it looks like it will fit just eyeing it. Does anyone have any experience with this?
 

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Yes, it will fit, but it's not a desirable swap for a number of reasons, unless you intend to modify it. It's a fairly lazy smog V-8 that has poor horsepower and rev-ability. An original 298 with a decent cam, carb, and headers would be much more original and a much better runner, in my opinion.
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You don't want it. The 400M shares the exact same block as a 351M, only the stroke of the crank is a bit longer. Pulled out of the car no one can tell them apart unless they tear them down and measure the crank. Fitting one into a '66 you run into the same fitment problems as someone trying to put in a 390. It will be VERY tight up against the shock towers. In 1967 they widened the Mustangs a bit mainly so such an engine WOULD fit. THough the M engines have a few design features in common with the 351 Clevelands (and NONE in common with 351 Windsors) they are very different engines. SImply stated, if you put a Cleveland in your car you could go to a 1/4 miles dragstrip and have fun, maybe even beat some people. With a 400M you might do very well in a burnout contest but actual race performance would be embarrassing. A 351W wouldn't be as much fun as a C but still would be enjoyable.
The really bad part of using an M engine is that they use the "big block" bellhousing pattern. Which means you have to use a big transmission. For an automatic that means a C6. Which won't fit into a '66's transmission tunnel. You have to cut and patch sheetmetal to make one fit. If you go manual that means a "big input" toploader four speed, if you can find one. And find the bellhousing to match. They're out there but not cheap.
Unlike a Windsor or Cleveland which will fit into your engine bay and use the "small block" bellhousing pattern. Meaning you can use your old stock C4 automatic, a more modern AOD automatic, a classic 3 or 4 speed, or even a newer 5 speed wihtout a lot of trouble (they sell kits).
A 400M is a decent truck engine. Makes lots of low RPM torque for it's size. Gets bad gas mileage. Is pretty big and heavy. And there are almost no performance parts for them other than four barrel intakes (all 400M's came with two barrels from the factory).
You can put one in your car, but there are any number of MUCH better choices.
 

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You might fit it in with modifications but it's not a good choice. It's less desirable and less serviceable than either a 351W ci or a 302.
 

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The 400 is an 'M' block, Modified, because it was a take-off on the 351 Cleveland block. The 'C' block had some bottom-end short-comings, so a fix was tried, which was the 'M' block. Didn't work. It's a good solid low rpm utility engine quite popular in the LTD II series, as a police & taxi engine. There are almost no aftermarket parts available for it, because it was only around like two years. You're best bet is a 'W' Windsor block, which includes the 289, 302, 5.0, & the 351W. Tons of parts available, & the 351 W can be stroked to 427 cubic inches.
 
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