In my opinion you would be much better off going through the engine you have.What if you buy another one only to find out it is beat.I would go threw the engine you have and you would know it was done right. and should last awhile.I am buy far no chevy guy but it sounds like the chances are you wont find 1 for $500 and if you do without going through it you are taking a chance.Call the junk yard,they will probably tell you they have one that only has 30,000 mile when it really has 130,000.Probably tell you that it runs great but they wont mention that it burns oil.