I picture would help greatly . Look at the valve covers . If it has 2 bolts on bottom and 2 bollts on top " 4 total " holding the valve cover on then it is a small block . Now if it has 4 bolts running along the bottom of the valve cover and 3 along the top " 7 total " then it is a Big Block .... Now that will help you pick the " family" of the engine but you still want to know the displacement " Cubic Inches " so here we go . As long as the engine block has never been " Decked " there will be a suffix code that can be decoded that will tell you what the engine came from , what the displacement is , as well as horse power ratings when new. The suffix code is located on a small square pad sitting directly in front of the passenger side head ' Usually hidden behind the alternator " Look at the pictures below to show you location ... At the end of the number there should be 3 letters , those are the suffix code . You can google the letters and see what turns up . A website known as nasty z28 has some really good info . You may need a flashlight , and some good cleaner to get a good look at the number down in behind there ........ Now if the truck has a big block , The number can be in one of two different locations , The first is the same as the small block . However i have also seen mid seventies big chevys that had a pad directly center , just above the timing cover and below the front of the intake that has the suffix code on it there ... If you are having trouble finding the code online and breaking down what it is , Just respond here with the numbers and letters and i will decode it for you .