It shifts at a very high RPM, did you install a special governor weight?!

Did you install some type of aftermarket linkage like Lokar etc.? These make often wrong kickdown settings so this would explain why at WOT it is ok and then at part throttle it's maybe not enough kickdown signal from the carb.
It's not easy to adjust it the correct way except you got an original type of kickdown linkage or the one from Bouchillon Performance...

But opening up that hole (best would be to remove the orifice to drill that hole up a little bit) will surely reduce this flare. And I guess you are very close to optimun since the flare is small and only at part throttle... at WOT it doesn't flare and seems to be perfect...

John Kunkel surely got also another good idea how to improve this...

How many springs are installed in the front servo? On the <1971 front servos there where versions with 1 and others with 2 springs.. the one with 2 as used on Hemis and 440 make the front band release quicker thus helping flare up.
I guess this was done since the HP 440 also only had 6 springs in the front clutch retainer thus applying the front clutches quick.
Today mods are going more into the direction of installing all 15 springs in the front clutch retainer and leaving the front servo with both springs...
But anyway... removing the big spring in the front servo can also slow down the release of front band. But will make more 1-2 shift bindup, on the other hand you surely installed a heavy spring in that rear servo? (and HOPEFULLY and stronger spring retainer!)