Tranny & Rear end ratios

[tfen2001]

I have a '64 C10 with 292 cubes. The truck tops out at 50-55 mph so I'm looking for help on how to get more top end out of her. I pulled the diff cover to determine the ratio and found it is a 3.73 ratio. At some point I'd like to drop in a 350 with a manual 4 speed. This maybe a dumb question but upgrading to a 350 4-speed won't change the top end speed, right? So, I'll need to go to a lower ratio in the rear end, right? That being said, I think this may be a 12-bolt posi but I'm not sure how to ID a posi. There are some small coil type springs in the carrier. I haven't been able to find any pics that would show whether this is a posi or not. With the rear wheels off the ground, both wheels spin in the same direction - does that mean anything? Thanks for the help.