Eaton 8LL Max Speed on Highway

What model 8ll? Looks like it could have a .74 or 1.0 top gear ratio.
Here is a chart to help find out what rears you want to run
https://independantparts.com.au/downloads/speedratio.pdf

 

Roadranger has a road speed calculator. You need to know the final gear ratio in your transmission and what your rear end ratio is and it will tell you what RPMs at what speed you are traveling.

Calculators

If you are running 1700 RPMs at 65 you should have no problem hitting 70 +.

If you are governed at 1700 RPMs you should be able to turn the RPMs up without any problem whatsoever. You could probably turn that engine up to 2400 rpms.

I would do that before spending a lot of time and money on changing your rear ends around.

I would also check your transmission to see if it's direct drive or Overdrive. It's a lot easier and cheaper to put an overdrive Transmission in if you do not have one already. And then you can do the rear seal and the clutch and all that stuff too.

 

You may want to fix that tach. I can do 65 in my log truck, but it is right up there at 1900. while I was waiting on a barge to get mine home last summer, the mill ran out of wood and loaned me theirs. They had swapped out the tranny and put a direct drive in it, 5 max, except you had to pay attention, it has a sweet running b model in it and if you weren't watching it would turn 2400 real easy and more to go.
What rears are in yours now.
What rears are in yours, I have a friend in Az that specs his with 4.33s, this gives him plenty low to get off in the soft stuff and he can run 70 on the interstate all day too, but he runs detroits too.