Bigrod1984

Hello all, I have a 1982 C30 Camper Special for a long time have barley used it but was my father in-laws that past away in 2013. We built it together from a man that had it dissembled, engine out, body off frame, bed off but all sitting in a huge building collecting dust since the early 90's. We picked it up around 2008 and had it together early 09 had it running and driving good and even got a tittle for it which turned out an odd ordeal considering it has a commercial title also vin number (1GBHC34W4CV11756) is weird not able to really decode it and has some really cool RPO codes. Anyway it's sat on the back burner for yrs and never put much thought into it but just recently got into looking at it and finding out how unique this truck is. I'm new to all this forum stuff don't usually post or ask questions i like to try and figure them out on my own. I'm in need of a little clarification on my engine i have. I took some pictures of the casting number and date code the best i could considering how much of a b**ch it is to get back there to it especially when the engine is caked in oil and dirt. The best i could make out is casting number 3999290 and date code D30 75 tell me if ya'll see anything i'm not. From my research casting number 3999290 is a 402 made 70-72 but if that is true how could the date be 75? Any help with the engine and vin would help. Thanks