I think the first step is to check the wheel size. Generally wheels are marked on the back side. You are looking for something like: 6JJ
Stock 1978 C10's came with …… ah ….. well, Tire Rack says 225/75R15 102LI, but Tire Guides says GR78-15's on 6" wheels. The equivalent to a GR78-15 is P215/75R15 (more or less).
A 235/50R15 has an allowable rim width range of 6 1/2" to 8 1/2". So we really need to know what the wheel size is.
But as a suggestion, try going up in aspect ratio. Not only are the 235/50-15's a bit small for the truck (load carrying capacity wise), but they are also small in diameter. I would suggest a 235/70R15 as it is quite plentiful and much closer to the original diameter.