E body to A body rear end swap

I have an ebody 8 3/4 rear end that is 3" wider than my duster 7 1/4 rear end that I'd like to swap. If I move the shackles in to fit, I'd have the big bolt pattern and an 8 3/4 in the Duster and only be adding 1.5" to the width on each side..... There must be something wrong with this brilliant (ha ha) idea. Advice?