Clanking noise
 

So I know this is going to be stupid but I'm no certified technician. I just bought a 1990 K1500 Silverado with a 350 5.7L V8 motor, which sat on a farm for approximately 10 or so years. So Far I've changed the spark plugs, wires, distributor cap and rotor. I did and oil change put a new thermostat and hoses on. I just ordered a new intake manifold because the thermostat housing bolts are stripped out. At any rate, this past weekend I was able to start it a couple of times then all of the sudden it just clanked, this made me think it was the starter. I went and got a new one and put it on, had the same results. One thing I'm thinking is that maybe the water pump is seized. The clacking noise is not enough to shake the truck but it's not a clicking noise either.

Any help would be appreciated.

Take off the belt and crank it to see if the clank is gone. If the noise is gone turn all the pulleys by hand to see if any of them are seized or hard to turn.

Belt
 

Sounds like a great idea. I had a thought that the water pump may be bad.