trans. swap, gear ratio or lower profile tires?

[67c10flatbed]

I have a 67 C10 that used to have a straight six with a four speed tranny, that was swapped out for a mild 350 and TH 350 trans. Well the long and short of it is that when I am traveling down the freeway at about 65 to 70 the engine sounds like it is about to let go. I dont have a tach so not sure bout the rpms. Not sure about the rearend ratio as well. My question is...would it be better to change the gear ratio on my rearend , swap out my 350 for a 700r4 or to get a lower profile tire since I am running a 31-10.50/ 15? Which would lower my rpm to where it will travel down the freeway without sounding like its on the brink of destruction?