Look what arrived from the mail today...
I have a 72 Demon and the same headers behind a 318... if it doesn't go in easy when you jack the engine you're going to have to pull it farther... Got mine up all the way then realized I needed an engine lift to get it in. That was with going in from the bottom and being way way off the ground. Good luck, let us know how it goes and I hope your experience is better than mine was!