>The 351C is pretty easy to distiguish from the 351m/400. The intake is different as is the (front) timing cover area and as you mentioned the thermostat.

No help - t-stat location is the same on 351C, 351M and 400 with exceptions for deck height. 351C will have a small block bell housing pattern and have a C4 or FMX behind it. It will also sit on regular small block (289 / 302 ) mounts. The t-stat housing will be the same height as the intake manifold seal surface on the block.

Anything else is a 351M or 400. Fin next to dist., motor mounts bolt to the block with three bolts, big block bell, 1" more deck height. As noted, the stroke on a 351M is 3.5" and on a 400 it's 4", so pull a plug and stick a wire in there to get a rough reading.

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1967 Falcon 4 door 351C - Owner built, owner abused.
70 Mustang 351C / 06 Ranger, 04 SuperCrew parts hauler