Re-spec'ing 07 387 with 06 isx 450

I wasn't sure which forum to put this in, so I'll try here for start.

I have 07 Pete 387 with 06 ISX 450 and 10 speed FRO16210C and 3.55 rears, E12021 front axle, DSP41 rear axle.

I run from MN to Seattle then down to Portland and back to MN every 8 days. I run at 64 mph which is at about 1440 rpm. I'm averaging 6.5 mpg. I run Michelin X one Line Energy D super singles. They're 40 inches diameter.

Before I was a company driver, I drove 13 speed. Loved that flexibility.

I want to swap out the 10 for 13. Perhaps RTLO16913A? I'm wondering if my 3.55 will be fine? I feel like I should go bigger like 3.25 or 3.05? So then i can go 70 and stay under 1500 rpm. How much will it hurt my mpg if I went to 3.25 or 3.05?

 

So I asked my shop how much to change from 3.55 to 3.08, it'll be around a 2,500 for parts. So depending on the cost of transmission, I should be able to do this for under 10,000 maybe under 7,000 if i can get a deal on a transmission.