extended radius arms....

[Sasquatch_Ryda]

I measured how much I extended the arms over stock, then measured back on the frame from the stock mounts to figure where my brackets would mount to the frame. Just so happened there are 3 holes on each side of the frame for the transmission crossmember in the perfect spot.

 

[4x4junkie]

You could measure where the axle is stock (or use Sasquatch's method), though what I did with mine was just mount the axle into it's brackets, then moved it as far forward as I could without the passengerside axle shaft/beam hitting up against the pivot bolt & bracket for the driverside beam. This ended up putting the axle right at about an inch further forward than it was originally (which actually improves the steering ackerman angle at the wheels to some degree).

 

[B2Guy]

so the thing to do would be hangin the new ra brackets before taking the axle apart maybe drop a plumb line and mark each hub where it matches up with your line?