Hello all. New to the site, lots of good info. I have a question about the engine in a 66 C10 I recently purchased. The numbers off the block are 3877178. According to multiple listings I've seen, it could be a 230 or 250. How can I tell the difference and are the majority of external parts interchangeable?? Thanks in advance for any assistance.

Go by the stamped number behind the distributor.

The difference between a 230 and 250 is the stroke. Everything is the same except the crank and pistons.

Also, the pistons in a 250 are dished, the 230 is flat top.

Most of the trucks and full size (Impala, etc) in that era came with the 250. The 230 was used more in the Chevelle and Nova in that year. But the carb or distributor number will let you know for sure.