GM used the same electrical parts across many truck model lines, so it might be the same fuse block as the pickup trucks and Suburbans of that vintage. By '78 I think they had switched to blade fuses, and also had the extra circuits for power windows and locks.
There are hundreds of GM pickups in the junkyards, and lmctruck.com and many other retailers sell replacement parts for this vintage of light truck. I would check those sources first, and compare them to what is on the grain truck. This site also has a lot of info about the light trucks of that vintage http://www.chuckschevytruckpages.com/