Overdrive in Early A

My Dart has a 904 in it now. The front half of the 42rh should be the same as the 904, correct? So the tunnel mods are from the crossmember back? No enlarging in front of the crossmember?
Look, the only way you're gonna find out is to make your transmission decision and do it. I'm not bein smart, that's just how it is. Each of these cars is a little different due to assembly tolerances at the factory, so no two cars are a like. Your car may require a little more modding in certain areas than another.......then again it may require a little less. It's simply something you're not gonna know about until you get there. You just have to make up your mind and do it. Nobody else can do it for you and certainly not over the internet. After over a year of getting parts together, I am getting ready to get the sawzall and angle grinder out and hack the transmission tunnel out of a perfectly good 64 Valiant to put a world class T5 five speed transmission in it. I've read all I can get my hands on and it's about time to just start cuttin. Once it's cut, I gotta finish it up. Same for you. You just have to do it. Nobody on here can tell you exactly how hard or how much you're gonna have to mod on your car. All we can do is give you a "close guess". You have to do the rest.