I'll throw my 2 cents worth in here because I kind of have these combos.
My 70 Challenger with 3.23 and a 225/70R14 rear tire runs 3300 @ 70-75 mph.
My 68 Charger with 3.23 gear and a 4 speed od out of a pu truck with 275/60R15 runs low to mid 2k @75 mph. Gets mid teen, mpg from a 440, all making it a pure joy on the highway.
You don't have to worry about the torque capacity of the od gear, because the countershaft is seeing the same torque load in every gear except 3rd which is direct, input and main shaft locked together. The overdrive actually removes torque from the mainshaft to the tires. Just as first gear multiplies the torque by 3.09 the over drive is multiplying by .72, so the theory that 300lbft can't be shoved through overdrive does not hold water.
The aluminum case is torque limited but I would not afraid of any of the iron case ones.
I have to go with RRR on the speeds. My transmission in my big truck is a 15 speed, I do not use them all, all the time but they all count. The high gear (5th) in each range is a overdrive (less than 1:1) but they all count and the 15 speed is represented in the model number of the transmission.
My 2 cents FWIW.
