After some delays I have made some great progress on this beautiful 1949 Epiphone Blackstone. Now that the truss rod slot has been refurbished and the truss rod installed, the top of the neck needed to be carefully inspected to see how much warpage was causing the upbow I was seeing before the project began. Using a straight edge ruler I measured a 0.040" "depression at the center of the neck. Thankfully, the upbow was nicely centered between the heel and the nut, so I am confident that any adjustment of the truss rod will place the point of maximum force exactly where it will have the most effect. After some thought about this project I decided to flatten the neck before the fretboard was reinstalled, this way, the truss rod would not need to be overstressed trying to correct such a large up-bow. At first I thought I would just sand the whole neck flat, then I caught myself and realized this would have been foolish, since I would be removing 0.020" at each end, compromising the neck at each end and losing some of the fretboard extension, since at the extension end, once the neck is flattened and the wood is removed, the neck gets shortened by about an 1/8".
So, the best solution was to do the real work and craft a maple shim, feather edged at both ends, with a center thickness of exactly 0.040". This wasn't as difficult as I thought it would be, just time consuming at the machine tools, however, the results were well worth the effort. Once glued down using the back side of a 16" aluminum radius sanding beam, the top of the neck is now perfectly flat.
The photo is not so great, but you can see the flat sawn maple used and the four holes I had to punch in the shim, so that I didn't cover up my alignment indexing pin holes.