I have a friend with a 67 Cuda with a stout 360 6-pak and an A518 overdrive auto driving a 8.25" with 4.1x gears. Not sure on the tire but they are pretty tall. I believe he gets about 18mpg on the hiway with that set up so it's got to be turning pretty low rpm's when in OD.
Anyway, the tunnel needs to be split down the center from several inches in front of the torsion bar cross member to several inches past the cross member. The cross member needs to be cut too. Everything was opened up a couple of inches and a new piece of cross member was welded in along with etra sheet metal. FWIW, he was able to install a factory console and use the factory shifter. You can't tell when the interior is in that this work was done.
I am running a 360 through a A833OD overdrive manual and a 3.55 geared 8.25 in my 68 Barracuda. I have 235/60-14 tires on the back and at 70mph on the hiway it's truning about 2000 rpm's.
BTW, the 8.25 that I have in my Cuda orignally had 2.94 gears with a sure-grip.