Engine902

Hi all, I have a 1980 x fire truck that I am converting into a pick-up, daily driver. It has E6-260 motor, Allison MT644 trans matched to a upgraded Airliner suspension running 4.10 ratio. The truck tops out at 65mph using almost full rpm @2000 - 2100 I'd like to get it to 70mph at 1600rpm for highway trips. It seems to love sitting at that rpm, but obviously the speed right now Is allot lower. Can anyone recommend a direction to achieve this, most cost effective first. Can my engine take a couple hundred more rpm's @2300. Lower ratio 3.73?, Overdrive? Tia for any help and advice Karl