If the engine is orginial to the truck; your vin number will enable you to determine the engine size, on the side of the block near / above the starter is usually your casi ng block number. It should read something like D0AE6016C something of that effect.
Ford part number started back in the 50's; the 1st digit is the 10 year era increment ( like A=40's, B=50's, C=60's etc, the second digit is the year with that 10 year increment (D0= 1970 model. The 3rd digit is type of vehicle A= full size car, T= truck. The 4th digit indicates wheither a engine part or chassis, E= engine; C = chassis.
I hope this helps; to determine wheither its a 302 or 289 etc is alittle more challenging.
The firing order will also weed out engine types as well.

Mitch