Rhodes,
You wrote " John Hunter was a PH of some repute in Africa for most of his life back around the 1920's & 30's. He wrote that his favourite double rifle for dangerous game was a 500NE weighing 10.5 pounds."
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If John Hunter was as sensitive to recoil as he said he was, he would have been much better off with a heavier .500 nitro; I can't imagine where he would have found a British .500 weighing only 10.5 lbs, as they averaged about a pound heavier, at an average, and proper weight, of around 11 lbs 8 oz. European double rifles, on the other hand, have always tended to be light for caliber.
Here are some common weights for .500 nitro sxs double rifles: Beretta 455 model, 25.5" barrels, a $55,000 rifle---11 lbs, 10 oz. Westley Richards droplock, 11 lbs. 15 oz. Even a modern American Searcy, who knows a thing or two about proper double rifle weight,---11 lbs. 8 oz.
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Felix,
You wrote "Leaving barrels at 24"
When dealing with a rifle that is already weight challenged for caliber, one want to add as much weight as possible, overall, within reason. Thus, I would take a hint from Beretta and go with a barrel length of 26" unless you already committed to 24".