I have seen pictures of these trucks with super singles, If I find it I will post it.
I have a 969 and drove it around 750 miles one way (41-1/2 mph) 2300 rpm's with 900-20 tires, Truck weighs 22,000 lbs , 6 mpg. I am going to put 11:00-20 NDCC on mine because they are easy to find and much cheaper than buying 10 new 9:00-20's, Do not run 9:00-20 8 plys off 2-1/2 ton 6x6's on the front of this truck they are to light, need to be 10 ply or more.
Keep the gearing original because if something breaks parts can be found and there are no taller gears to put in place, the transmission is all ready 0.768 to 1 overdrive.
The Hercules RXC 529 cubic inch engine is 131.5 HP. torque is 396 foot pounds at 1000 rpm's, You can still today buy a brand new RXC for $2600.00
Some of the guys in Europe have put Cummins diesels in place. I do not know much about Japanese diesel and gear boxes , There is none here in the states that large and you are going to match or have more torque than stated above.
Just remember these trucks are highly collectible and hold a value when original .
Can you post some pictures of what you have.