I believe the flex plate only goes on one way. Yes slips in all gears. I changed to a Lokar kickdown. Standard valve body. It will seem like it’s got grip at lower rpm but it will only move slowly needs some revs to get moving at all. I backed it up I gear with no real throttle needed to move it. Yes fluid is at proper level.I guess you checked the fluid-level in Neutral on level ground?
And the KD system is properly adjusted,
And the flex-plate only fits one-way right?
Details would be nice., as in
Manual valve body?
slips in all gears, or
just in forward gears, or
only at WOT.