Welcome to the site. Just a few questions. Is your GMC made in Canada? If it is, you may well have a 235. If it's a low pressure engine, it won't be worth much, in fact, I took mine to the scrap yard. How do you know it's the original engine? Lastly, do you know the condition of the engine? Just a few variables there, but as mentioned, check what others are fetching. Good luck with it.
Leo
