Sounds like governor pressure is too low in relation to TV pressure. Classic 700 problem, esp at WOT. I agree with Crosley, try the OE TV plunger sleeve. Also, look hard at the governor and the bore in the case. The OD of the gov lands must be smooth and consistent. Memory tells me .805 is the desired OD, i might be off a little on the number. Check the gov drive gear and retainer pin. If the gear is loose, it will leak gov oil and cause upshift concerns. It must be a mild press fit to seal in the end of the gov. Also, many times when the gear is replaced, the new one is scored by the pin hole (inside burr) when the gear is pressed into place. NO BURRS. Check the gov head (weight area) to see if it is loose on the gov shaft. Seen this a bunch also.
The TV and gov oil circiut on 700 can cause hair loss. If you could see me you would know. Mostly, gov springs change WOT shift timing, weight changes low speed. They DO work together which adds to the complication. After you have checked all the above stuff I would make a significant spring change to the gov.
If you want to tune individual shift timing, then you change shift spring weight. Keep us informed how changes go. Sometimes what seems insignificant to you may hold the key for others.