Most B/E body axles can't be shortened to A-body length. Which is not to say they can't be shortened, but the location on the B/E axle you need to make it A-body length isn't large enough in diameter. You can make most of them shorter than A-body length because they step back up in diameter.
To shorten a stock axle shaft to A-body axle length the axles pretty much have to be C-body or truck axles.
Yeah but then you'd be stuck tracking down a complete set of small bolt pattern rear brakes, because not all the parts you need are reproduced. And unlike the large bolt pattern rear drum brakes that were used all the way up into the 90's, your donor list is real short.