to get the casting you can use clay to imprint the casting then hold it to the mirror. That tag does mean it has been bought as a rebuilt from probably ither GM itself or a big rebuilding/ selling company, which could be anything from Napa to some large internet sites. If you can find the engine pad, which is the left side looking at the motor, and it is right under the head, it should have some numbers and letters on it along with a partial vin, it could have been machined off when rebuilt but the first 4 letter code would tell what it is if it was still there, you would need to go to www.nastyz28.com to get the conversion tho. If your alternator is on that side then it is right underneath but it should be visable, it is a flat horizontal spot approx 4" long and approx 2" deep.