Rear end upgrade??

No tach unfortunately, but I have good news. I WAS able to find the tag, It was hidden under the paint so I never noticed it before. I guess I really lucked out, I've got 3.21 gears in an 8 1/4 that came with a slant six. So when I get the 318 in, how much improvement would I feel in the butt dyno with a sure grip and 3.55 or 3.90 gears? or would 3.90's be too much? I've never had the car on the highway but I'd like to take trips with my grandpa in it so I need it to cruise good at 70 mph.