Lol, I have this same problem with my 77 280. So far I have replace Exhaust, Fuel Rail, Fuel Pump, Injectors, Crank Timing kit, FPR, and now I am putting in the XR300/PS91 setup to see if I can get it to go away.
Truth be told though, I'm pretty sure the issue is Fuel, at least in mine. I let the tank get really low while I've been doing Idle testing after all these part swaps and the one time I took it out to go down the street, I went WOT and (Remember the low fuel) when I hit about 3k it did the exact same bog that happens at 5500 normally. So then I accelerated slowly and no problem all the way up.Right around the same time all the low fuel would have been stuck to the back of the tank.
I know this is an old post but if any of you figured out exactly what it was, lemme know. TPS, AFM, etc...