I have an 89 1500 it came with a 305 stock. The last owner put in a new motor. I thought it was a 305 but after thinking why would someone put in a 305? they are more expensive and less common. So now I am thinking it is a 350 is there any way to tell? what do you guys think?

 

Im not sure where exactly they would be but there should be numbers on the block somewhere. If you find them, look them up (might be able to do it on the web). Worse comes to worse call GM with the numbers. Someone might have a better idea tho. The only way I could easily tell with my 91' was by the size (compared to another truck) cause they are almost identical otherwise.

 

other than the casting numbers you could measure the bore size 305 is 3.750 and a 350 is 4.00 bore. I pretty sure the 305 also had a smaller harmonic balancer on it.

 

Look on the cylinder deck, where the heads sit on the block,on the radiator side of the block. I think it's the passenger side, you will see a series of numbers. Use them to find what engine you have through a web search. If you have any trouble with the web search, post the numbers and I will look them up.

Changing the motor to a 350 will depend on where they got it. If they used thier old motor as a core it is most likely still a 305. Most rebuilt engines from suppliers have been bored, hence, your 305 may be a 307. 350 may be 355.