I would and did order my Moog brand UCA's through Summit Racing. They can be founder for less elsewhere and then you can get them price matched and free shipping on top of that. I do most of my parts order through them or NPD but Summit typically has most of the parts that I need. If they don't, I generally go through NPD as I've yet to have any issues with them. In terms of the UCA's being centered, it's the two stud bolts that come out of the UCA and go through the shock tower that need to be centered. If they aren't then it won't be sitting evenly when bolted into the shock tower. A couple of issues with this can be, and I had one of them before replacing my original UCA's, is that it can make the alignment off, more difficult to align, having the UCA too far towards the rear of the car if you do the Shelby drop and the issue that I had was that the UCA was rubbing on the inside of the shock tower because it wasn't centered properly. The UCA outer nut was rubbing up against the inside of the shock tower and it also doesn't allow you to grease the insides of the UCA's. I bought Moog replacement UCA's from Summit that were centered and then went to the hardware store and bought some 90 degree zerk fittings to be able to grease the shafts and UCA's much more easily and not have to drill holes through the shock towers. It's also better peace of mind to have brand new UCA's instead of just replacing the ball joints because the shafts are likely not centered or are worn out. I'd spend the extra $30 or so per side and have brand new UCA's instead of just ball joints. Getting new UCA's that aren't binding will probably even help the ride comfort/quality and if your mechanic is taking the UCA's out in the first place, I'd have him drill the holes for the Shelby 1" drop while he's in there.