Transmission slipping

You said;
"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."

In this case, I'm with RRR,
I think the Forward Clutch packed it in.


But before you drop the trans, I think it would be prudent to check the Manual-Valve adjustment, to ensure the trans is truly in First when the shifter says that it is supposed to be.
If you have never done this before;

Jack and block the car up high enough to get under it. Then
remove the shift mechanism off the trans, and
check the manual valve adjustment; First pay attention to the detents, and listen for the Parkrod locking and unlocking the driveshaft.
If the Manual valve is between detented positions, she will dump the fluid into the pan, and nothing works right.
If that all works;
With the shifter removed, jam the park-brake on, then jam the trans into park by hand, then start it up; then, by hand, click it forward past reverse, past neutral, and into manual low. If it jumps the driveshaft, the trans may be working. Manually click the shifter back one click into neutral, then go Shut the engine off.
With the engine off, move the shift selector into Neutral. Now slide under the car and see how far out the rod is from dropping into the trans lever. Adjustment is provided to synchronize them.
After the lever drops on and has been secured, Go check that the shifter still goes from end to end, then put it into neutral and start it up.
With the Parkbrake still on;
try Manual-Low again.
If it jumps the driveshaft; then try Reverse. If that also jumps the driveshaft, put it in Park, lower the car, start it up and see what, if anything happened.
If no forward motion,
then
drop the trans.