High RPMS

[69ChevySB]

Hi there folks, I was wondering what I could do to get lower rpms at higher speeds....when i'm goin down the freeway or highway it sounds like my motor is gonna explode because i'm going like 75 and my rpms is like 3500-4000 at that speed...what can I do to prevent my rpms from going so high? Maybe it's because I have a 3 speed on the floor and it feels as if I need another gear to throw it in...would it make a difference if I put a 4 speed in, or automatic transmission? Thanks.