It sounds like it, the shifter or valve body selector, is not in each gear properly twocents scope
Take the pan back off and put the shifter in neutral and then remove the shifter selector from the arm on the valve body and watch the manual valve to see if it moves when you lift the shifter cable clevis or shifter bracket off the selector arm, if it moves you need to adjust the shift linkage so it doesn't move when removed in neutral, first, second and high gear scope
Let us know what you find out thumbs How good is your converter? What brand and foot and flash stall rating RPM?
Good look on this thumbs

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)