Looking good! Real good. While you have it in the chuck and centered, mark the chuck jaws so you can get it back real close when you need to put it back in the chuck. You might also use a large cone center in the tail stock to help align it with the headstock when you are installing it in the 4J.
I made my own mandrel and bushings.
I bought a piece of 1/2" precision ground shafting to use for a mandrel, and a 1/2" chucking reamer. Using the reamer to finish the ID and the compound set at a 2.5 degree angle to permit accurately setting very fine depths of cut I made my own set of bushings in .0002" sizes to use for aligning receivers in the fixture.
How the bushings are made is important. It is critical to have the ID and OD as close to perfectly concentric with each other as is possible. I chucked the raw material (12L14) in the 3J, center drilled and then drilled the rod 1/32 undersize, then used a boring bar to enlarge the ID within a thou undersize with the surface being perfectly concentric to the lathe spindle centerline. The drill will wander around, the single point cutting boring bar assures the hole is straight and true and centered on the lathe axis so that when the OD is turned it is perfectly concentric with the ID. Then with the lathe running as slow as it will go using lots of cutting oil I carefully reamed the hole to to exactly 0.500" which made it a perfect barely sliding fit on the precision ground shaft which measured 0.4995". After reaming and before turning the OD I checked it with the same .0001" test indicator I use for aligning barrels in the headstock. I only had one that showed any morement at all following this procedure - I cut it off and made another one which didn't show any movement. I checked the OD for concentricity with the same indicator after it was turned and polished.
The OD of the bushings in the set were turned to diameters spanning the useful range (I forget the exact dimensions) in increments of approximately 0.0002". It took me a while to make the 12 or 14 bushings but they work great. With the shafting between centers I can spin a bushing on it and see essentially no indicator movement which is what I was looking for.
Greg's mandrel and bushings are world class, hardened and precision ground to perfection. I just couldn't afford them, and since I work with Savages I don't need to ream the bolt bore and sleeve bolts, so I made my own mandrel and bushings.
I concurr with the suggestion to make a second set of holes for short actions. I did that on one of the two I made.
I put the original set of holes too far back so I ended up with three sets of holes on the chuck end. The shorter fixture is used with an internal collet like this:
Looks to me like you're rolling!