It would be nice if OP's would relate such details when asking a question. He didn't say whether the engine or the tranny was original to the vehicle. For sure get a later slant. A "correct" engine would be a ~1978+ w/ "peanut" heads and hydraulic lifters, though a ~1968+ would be a direct bolt-up (larger crank hole). I saw some military-surplus rebuilt slants ~1980 in wooden crates sell for ~$300 a few years ago on ebay. Many people found the slant under-powered in a truck, especially for towing. It might not be hard to swap in a V-8, since a factory option. A 5.2 or 5.9L w/ it's better tranny is fairly interchangeable and works better (MPFI, overdrive, ...). They are common at PicNPull.