you would want to use the 351w. the M is a bit on the long side for a cj. my personal fave is the cleveland, but i would only use it in a mud, or other racing machine as they are built for high rpm usage. the windsor has smaller heads and built for more low end torque, and they are not muck larger than a 302, as well as share the same mounts and bell housing pattern (the cleveland shares the same mounts and bell pattern, but is a longer, heavier smallblock). the M series shares the big block bell pattern.
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