I found that I could build nukes as soon as I got rocketry and uranium. I did not have to wait until the Manhatten Project was built. In the civpedia it still says I need the MP to build it. I can't find the Manhatten Project XML file so maybe its a glitch? Currently playing with Roosvelt on a graphically modified version. There are a few simple XML changes I've made but I haven't touched the ICBMs. I did mod a chinook helicopter onto Rocketry tech as well.