Using 8 3/4 with /6

Unfortunately I didn't build this myself and I have limited information on the engine specs but I do have this link, Mopar 225 Cam & Kit 268 Mechanical this is the set up that the guy told me is in it, it was converted to big bolt pattern, and I believe what I said, "it can't put the power down", isnt the right thing to say which is what some people are confused by it but what I mean is that if I give it full throttle from a stop it is obnoxiously slow and think this is due to the rear end which is why I am looking to swap it. Please correct me if I'm wrong on any of this because I'm still learning.
You need some gears. AKA, more gear, higher numbers. (I'd still go for the 8-1/4)