[oil pan 4]

They could be testing for the highest BSHC (Brake Spesific Horse Power Consumption).
That is when you calculate how many how many pounds of fuel per hour it take to maintain each and every horse power for an hour.
They were likely testing under heavy load.

You are going with a set up that sounds like what I want to do with my car.
Big displacement engine, that makes big torque on 6-speed and looking for great MPGs.
In my car I would like to install a T56 with the new engine and swap out the rear later after road testing the 3.08s (It also has a 3.08 rear end). I would like to see what happens in top gear at 65mph turning 1150rpms.
I would like to try for over 30mpg on a 500hp V8.

I am guessing you have a 6 speed with a .7ish top gear, 4.10 rear end and 31'' tires.
The cheapest way to change your final drive ratio is to swap rear end gears.
If you replaced 4.10 gears with 2.73 (or what ever is close to that in the dana's) you would drop your RPMs from about 2000rpm to 1350rpm.
Or if you went with 3.08s you would drop your engine RPMs to 1500 at 65mph.

Yes you can tow with that.
I was able to pull 3 tons with my old worn out 6.2L on my current trans/diff/tire setup.
I am not going to be doing any heavy towing like that any more, so after my turbo goes on I am considering going from 3.08 to 2.73.

Also remember the bigger the rearend gear teeth the stronger the ring and pinion are. A 3.08 is stronger than a 4.10 because the 3.08 has big knoby teeth compared to the 4.10.

Correction above my truck engine turns about 1300@55mph and 1550@65mph.