B body 8 3/4 in an A body

If it really is 54 1/4 then its a 65-67 B rear. That's narrower than a lot of the B rears that folks put in Dusters, but its still going to move your wheels out over 3/4" per side. 14x6" is pretty small, but so is a 3" backspace. Typically the B body rear swap is used to allow for modern, high backspace rims (5"-6" backspace). You'll have to take a look at your wheels and see if there's an extra 3/4 to 7/8" of an inch before you get out into the quarters, you may need new rims.

The perches will also be at 44", you'll have to move them in to 43" center to center to make it work.