i have a 74 scamp, with a fresh built 318. it has eagle forged rods, lunatti cam, 360 heads, and a 727 trans. The comp ratio should be around 9:6:1. I was looking for a 3.91, or 4.10 rear for it. I am a little confused, can i just change the gears? or do i need a whole new rear end? if so which...