E-body 8-3/4 banjo, 489 case and a set of Yukon cut-to-fit axle shafts. How should I go to find correct length for the shafts? Standard bearings for adjusting preload. I know the factory length can be found but would like to take any abnormals out of the picture.

Contact Yukon tech for the correct procedure for them

Ok, that is sure a good advice.
I have everything on the work bench and what I think I am looking for is the position of the thrust block inside the carrier and how much play each axle shaft should have against the block.
If I should guess, the thrust block should be sentered in the carrier, left shaft barely touching the block, and right axle (adjustable side) having about max (.0018) endplay to the thrustblock with the same distanse between axle flanges and housing flanges at both sides. This will not allow for much tightening of endplay after things seat thou.
I'm sure some of you know the procedure:)

Cass,aka doc diff told me if it is a stock factory housing use the factory length. If housing is narrowed, or retubed like my dana 60 was, stock may not work. The axles need to be machined flat on the end not just chopsaw it off. A tape measure stuck down is not good enough. Also do not forget to add brake backing plate thickness.

Other issues that can creep in, make sure center section has gears in it so diff is sitting where it needs to be, dont just center the spanners and call it good. At that point you should be able to center the thrust pins, I centered them with washers machined to fit. We made a tool to measure out of a piece of pipe with cap on it, machined flat, and a threaded adjuster on end.

So you have cut to fit axles and still use taper roller bearings?
Your life would be much easier with new style radial ball bearings and snap ring retainer plates

Let the flaming begin about the life span of ball bearings

Gus