71 Duster 340, 727 Manual Valve Body - question...

I do not think it is a shifter problem.
What brand of valve body is it?
Is the band applied in first gear? Some are and some are not. If not it will free wheel in first gear. One way to find out is to lift on the throttle WHILE in first gear. If it free wheels then the band is not applied, If it drags you down, like a down shift then the band is applied.
NOW THE BIG QUESTION.
Why in the hell are you down shifting while you are moving?
That went out with four speeds and glass packs.