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.