3 speed would like lower rpm's hwy?

Hi,

I have a 1968 DartGTS 340, whats the cheapest way to get lower rpm's,
Tires/axel would that really make much difference? I have 391 rear end and don't want to change that becuase i do take it to the 1/8mile track. Anyway any ideas would would be great help to this weekend warrior!

Thanks Tom
And Merry Christmas