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The w block will be similar sized to your 302. The M block is substantially bigger. If the car is relatively clean, it should be stickered on the air cleaner or valve cover. Worst case, ask the owner.
Look at where the thermostat housing connects to the motor. On a 351W the thermostat housing connects to the intake. On a 351M the thermostat housing connects to the motor block because no water flows through the intake on a 351M.
If its a 351w then the motor will bolt to the trans and engine mounts. Because the 351 is a taller wider block than the 302 then you need the brackets for the alternator power steering AC from the 351 motor.
Will the exhaust have to be altered any to fit the 351?
i had to true dual exhaust on the 302.
And what about would the horsepower difference be the 351 was just rebuilt and has a police intercuptor cam and crank bored .30 over
Don't know for sure about the exhaust, but the 351 is wider at the top where the exhaust manifolds bolt on due to the taller block. If you have X pipes or crossover pipes then they might not be wide enough without alteration.