Looks like it could use 5800-6000 converter. The actual shift will need to be completed past 6500, but how far? Should have kept on going at the dyno. Think about rpm fall back on the shift. If the shift occurs at 6800 with a fall back of a 1000 would put you at 5800. More than a 1000 is too tight in my opinion. A second reminder is the Turbo Action (and I would imagine most stock based valve body castings) take about 500-700 rpm from the time the shift light comes on until the actual shift is completed during the 1/2 shift.
Doug