Pull a spark plug and insert a dowel rod. Have a friend turn the engine over by hand. Make a mark at the lowest and highest point of the stroke. Measure the recorded stroke. A measurement of about 3 3/4" is a 390, a measurement of 3.5" is a 352/360. The 360 has a 4.05" bore where as the 352 has a 4" bore. No way to know bore for sure without removing the head. Unless you know the truck there is no guarantee what is in there is what it came with, so a vin is almost useless for IDing your motor. I'm guessing you've already easily seen that its an FE by the intake.