What's the difference?

[Mrturner1]

So I have a 1970 C10, and it came from the factory with an L6 and 4 speed but somewhere along the lines it got swapped for a 355/th400 setup by the previous owner. I got an oil pan gasket for it yesterday and the arch that goes over the timing cover wasn't big enough. I took the gasket back (fel-pro permadry btw, the blue one peice) and ordered one for a 75-85 Chevy small block and it fits perfectly. What does this mean for my block and rotating assembly? Is it gen 1? Something else maybe? The crank journal is clearly bigger