I currenly own a 66 GMC 1/2 ton with bone stock original 250 inline 6 engine. The other 2 engines that were available were a 305 V6 and a 351 V6. Since the only inline 6 that was stock was the 250 you can tell for sure if this truck came standard with the 250 by the VIN. If it begins with an "I" then it came with the 250. The VIN plate is on the drivers side door jam at the very top of the jam. The 6066 site has all the information to decode the VIN. Also you can check engine spec tag which is by the drivers side air vent near the e-brake handle. It will give you specs on the stock engine.