Tazshido, I missed the part where you posted the VIN off the frame. That tells us a few things. The portions of the VIN stamped on the frame are the year, assembly plant, and sequential number. So, if what you posted are truly the first two items in the number then you have a '66 frame and the original truck was put together in St. Louis.

Now, you still have a GMC engine that has apparently been grafted in some time ago. You will still need to get the numbers off the engine to get that nailed down on a year. Also, if you want to better ID the front and rear differentials it would help to post some pictures.

Finally, the original VIN plate on the cab should be in the door jamb. Open the driver's door and look straight up at the jamb. That's where it should be on a '66 cab.

1963 Chevy C20 Longbed Stepside
1978 Chevy K30 Custom Deluxe