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.
That combination is going to need something between 3:23 and 3:91 to have any get up and go. Considering he swapped in a BBP 7.25, there is a good chance you are dealing with a gear set in the 2:4 range right now.
Also, and EXTREMELY important...you need a converter. Something in the 3500 to 4000 rpm range.
Lastly, I have found that most 4 barrel carbs are factory jetted way too fat for the Slant.
Pull your plugs and give them a look over, then consult the experts here.
We'll work with you, man. Patience and you'll get it all sorted out