What size engine?

I bought a pickup truck and the engine in it is not original. The engine in the truck now is a V-6. The truck is a 1976. There was no V-6 available in pickups at this time. How can I find out what year the engine is and vehicle it came from.

first of all you did not tell us what transmission you have ..auto or stick ?? i going to bet my money on the engine being a 4.3 becuase the 4.3 will bolt right up to any 5.7 chevy small block tranny ...all a 4.3 is ..is a 350 v-8 with 2 cylanders lopped off..also they make manifolds that you can replace the stock fuel injected v-6 with a carberator ....later randy

Look on the engine near the top of the block. Either on the front or side there will be numbers stamped in. This is the identifying mark of the engine.

in 1976 wouldnt it be a strieght six?? 250. 2 bolt main!
I highly dought somebody put fuel injection in a 1976...and it doent madder if it was auto or five speed...some people talk to to make the self feel good!