With the B52 set-up you could dial in axle location. Getting the axle located isn't going to be the hard part of your build.
I wouldn't assume anything, and to go further, even if there is a lip...can you be sure it's hard up against that lip after being rotated? My thinking on kingpin to kingpin dim's is for confirmation purposes. It's not likely to get one or both C's slightly cocked, or not against the lip (if it's there) but having KP dim's will put your mind at ease.
Once you're satisfied the C is exactly where you want it tack in 4 places 90 degrees apart. Re-check all dim's. If everything is right where it should be burn about a 1/2"-3/4" fillet into each tack. Ideally you would stand the diff upright so you're welding in the horizontal position.
Pre-heat the C to 250 F. I would weld it in a back step pattern. Weld 90 degrees of the circle (3 o clock to 6 o clock) then the next weld would be from noon to 3 o clock etc. Welding it this way you finish a weld by burning into the start of the previous weld, helping to negate a big crater at the stop/start. This method also greatly equalizes the heat affected zone, and reduces distortion a fair bit.
Interpass temp shouldn't exceed 350 F. You should ideally have two temp sticks, a 250 and a 350.
Bit of an oversight, but what is your welding set-up? I hope you have something with a bit of jam to it. E70-S6 microwire in .035" would be fine, but it does depend on machine. 7018 stick would work too...
Likely you'll do three passes to get to the appropriate fillet size. I'd aim for a 3/8" fillet. After welding, let it air cool if ambient is not too cold. Cast steel isn't anywhere near as fussy as cast iron, but it can warp. Even "through heat" is the key to keeping the C's true. If you're worried about it, you can throw a welding blanket over it to slow the cool down time.
Hope that helps a little.
