1968 C20 transmission swap advice

['63GENIII]

While the 4500 isn't a real straight forward swap depending on the transfer case combo that you have, it really is a cool set up. I have 4:11's with 35" tires and the truck used to howl down the freeway at 70 - 75. It just kid of lopes along comfortably now. I went to the 4500 from a TH400. I also had a divorced T221 at the time. I now have a divorced NP205. Huge difference. The truck seems to run around 2200 - 2400 where as it used to run around 2900 - 3000 rpm.