1987ZTPI

i bought an aluminium driveshaft a day ago from an 87-88 trans-am and i see that the u-joints on the aluminium 1 are smaller and the cups are smaller diameter than the steel u-joint driveshafts cups, how do i get this to work with my set up it's an 87 iroc-z that came with a steel driveshaft , what do i need to do to make this work i was told it was a direct fit.