Theres rough idle, and theres rough idle

I was just having this conversation with my son because he needs to get it a thermostat in that damn Power Wagon in the 2 degree temperatures in Denver. But he's got to get one the bigger one I believe 4 the 340 Cuda 4 Barrel manifold I put on there. It was originally a 2 Barrel manifold.. watch Rusty come in and bust my nuts and know that the same one will fit both LOL but I thought there was a bigger and a smaller one?