Need some advice with my 67'!

[lawjack]

Alright so this summer me and my dad swapped in a 350, and kept the old 3 speed trans. I have no clue what my rear end ratio is but my best guess is 3:73. The rear end is not original to the truck, back in the 90s my dad had this one swapped in when his old one crapped out on him. At 60 MPH im turning about 2800-2900 RPMs. This seems pretty high to me, i need some advice on which direction to go.. A new trans or swap rear end gears. I do travel lots of interstate miles because i have to drive about 220 miles to college every now and then, and thats not too fun at 55 MPH.