Gear ratio speed question

[5Tractorguy]

Got a quick gear ratio question.

I have a '61 GMC 5000 with a NP540 trans (direct drive in 5th) and split rear. 9.00-20 tires. Right now I top out at 35-40ishmph, 30mph cruising comfortably. My gears in the differential are 6.50HI and 9.04LOW.

Is this speed about right for what I have in my truck? Anyone have one similar? I realize it's old and all but it would be great to get at least 45mph, but I think that's going to take some serious gear changing to do.

Any help would be appreciated....