Much better. Standard steel plate does not come in .20 or .23. We will call it 3/16 (.187) and/or 1/4 (.25) Your measurements are accurate, just tolerance, burrs, fabbing dents, etc. Good job.
The "calculated" inside frame to inside frame is 35.64. The O/A measurement for crossmember you gave is 35.625. So that's close, like .0075-.0000 per side close, but will be OK with a little trimming.
The actual CL to CL number for the new axle brackets is a little over 38" if they are supposed to match the frame brackets CL to CL. The mockup will self locate the parts. The fact that the frame brackets are slammed against the frame sets up the alignment of the axle brackets. An exact 38" is not necessary. Probably will end up between 38"-38 1/4". Then the rest of the parts are aligned from that portion of the assembly. Don't forget the pinion angle requirement. Piece of cake.
Do a complete mockup using every part. Every bolt. Before final weld. That way you spot little things you didn't think of. I.E. missing parts, wrong parts, extra fabbing, other crossmembers, any trimming, brake lines, wheels, tires, exhaust, bed mounts, coil spring clearance, wiring, ebrake, stance, symmetry..........
If you use locknuts, use regular nuts for mockup. Then the locknuts at final assembly. Read both 47-54 and 55-59 online instructions as you go, to get tips.
Regarding the lower mount for the shocks. Did they give you the 55-59 3 hole extension tabs or the 47-54 style? you will need the 55-59.
Looks like you are all set for a great outcome once you get the new axle brackets.
