A 3.07 ratio diff won't make enough difference
Assuming of course that it doesn't already have that.
I looked at retrofitting either an overdrive or a 4 speed box to my 411 (same chassis as the 412) and it was going to need modifications to the floor of the car plus a shorter prop shaft, so decided against it. I also embarked on modification and injection of the original engine, which I now regret.
With hindsight I would go with a modern injected Chrysler engine with associated box. I believe this is what Bristol has done this numerous times so you would be doing something that the factory has already done. Take a look at the 411 "S6".