1972 360 Duster

Oops sorry, I see you have reverse as well. That means the rooster-comb is fine. Having reverse means also that the hi drum is and the L/R band are working.So with manual Low working, we know the forward clutch is working and at least the over-running clutch is fine. So out of the 5 elements, the only one missing is the KD band. So I would start with a band adjustment to be sure the struts are still engaged.
Is manual low in the correct place on the quadrant? I mean, are you perhaps starting out in 3rd gear with a manual only, reverse pattern VB. That would certainly explain the sluggish acceleration.