I have a 1986 D250 that I'm working on. For many years I thought it had a 318 but know I'm starting to wonder if it could be a 360. The guy I bought it from said it was a 318 but I'm not sure if he even knew. How can I tell which one it is?

 

On the driver's side of the engine block, just under the head, is a casting number. The last 4 digits should be 318-X or 360-X, that's your engine size.