I have installed several sets of Hooker street and race headers in 1970 E bodies, 1970 Cuda and Challengers. I removed the drag link on the steering from the idler arm and the pitman arm and one front steering spindle and laid it out of the way and then installed the headers from the bottom. That was easy to do with a stock Hemi K member, I'm not sure that the header tubes from cylinders #2 and #4 cylinders will clear the adapters mounts you are wanting to use
I did try laying them in the engine bay first before installing the motor once but decided to do it from the bottom after that
Good luck, I call them motors Hemiroids for a several reasons