What’s the part name for exhaust and size

I’ll admit that I don’t do much exhaust work other than cutting welding. Since I’m doing a custom build on my old D100, I want to attempt to build an exhaust system. Nothing fancy, just dual and maybe a crossover, if I can fit one under. I need the pipe that connects to the bottom of the exhaust manifold. It’s a 78 440 mild RV cam. Where and what size pipe. This thing is low to the ground. Thanks