I can confirm the frame is certainly a 1978.
If you are going to move forward keeping your project a 4x4, then great - if you are going to convert it to a 2wd, then you will need to deal with all of the crossmembers and mounts from the front cab mounts forward. The 4x4 frame for these years is slightly different on the top rail next to where the engine mounts are and will require modification (it won't hurt anything.)
Unless your project is a style side (fleet side,) you will need to ditch the saddle tanks and locate one (with ALL associated mounting brackets) from a Blazer or Suburban for a rear-center mount.