Once you have a good source of vacuum, the rest is adjustments. Vacuum modulator must be set to give firm-to-hard shift with no vacuum applied, then the vacuum proportioning valve at the IP is adjusted to soften the shifts at part-throttle.

First, try disconnecting and plugging the vacuum line to the transmission, see what kind of shifts you get. If there is still a flare, the modulator may need adjusting to produce more pressure.

Best Regards,
Jim