Remember that vacuum advance goes away under heavy throttle because manifold vacuum drops to zero (or near zero...there's theoretically a chance that it's adding a few degrees of timing if there's a bit of vacuum in the manifold). Either way, it seems like you have quite a bit of total advance (51 and probably climbing-12 or so vacuum advance=39 degrees). You said your total advance was all-in by 2000 rpm, but your numbers at 3200 rpm seem like they are still climbing. Excuse me if this has already been said, but have you done anything with the distributor advance curve?
Nothing has been changed on the distributor, it was just dropped in. I'm not 100% sure what brand it was, but something like this: https://www.topstreetperformance.com/hei-buick-nailhead-v8-distributor.html
