Governor could be too light in the weight and spring department but more than likely, there's a kit in the valve body requiring more line pressure to time the shuttle valves. Or valve body has a 1-2 shift valve stuck or a higher spring rate for that valve. Looser clearances in the shafts and clutch packs requiring higher line pressure. Line pressures could be down. TV valve is hung open bleeding off pressures, or a leak within the fluid curcuit of the TV system.
What kind of modulator is on that thing? Pull the vacuum hose and see if there is an adjustment screw inside the nipple. If there is, back it off (counter clockwise) to get an earlier shift.
Was this rebuilt by somebody recently or does it have some mileage on it?