Another option is to use a small block 727 and a Gear Vendor's overdrive. That is what I have done in my 73 Road Runner. In theory, you can split each gear to make the 727 effectively a 6 speed, but I have not found that to be realistic. 90%+ of the time, I simply use it as an overdrive once I get up to highway speed. But it has some controls and you can have it automatically or manually shift itself. So the bottom line is it is flexible in how you use it. If you drag race it, then I have been told shift 1, 2, 2 OD, 3, 3 OD (although the final is likely not needed). I have not drag raced mine so I can't comment on how well this works.
The GV overdrive is not cheap - it costs just under $3000.
As far as installation:
- They will tell you that you only have to minorly dent your transmission tunnel with a ball peen hammer or similar. I found I had to dent it more significantly. However, I did not need to cut or remove anything from my transmission tunnel. I merely gave it a good whack or three and in it went.
- You have to shorten your driveshaft. I found this to be very easy to measure and quick to have done.
- All other items fit in their stock locations. Of course, you need to install the controller inside the car and run a couple of wires, but I did not find that to be a big deal.
A well built 727 should easily be able to handle the power of the Hemi. The GV overdrive can handle 1200 horsepower out of the box, so I think you'll be OK there.
Here is what you get for a 727. The kit is 100% complete and comes with everything you need to install it.
Here it is installed on a 727.