In the past I've swapped steel to aluminum oil pans on both H/C and W body cars. Recently I almost suggested to a W guy that he buy an H/C pan by mistake. The mount under the harmonic balancer on the W body prohibits this and vice versa on the H/C body.
Here are the differences in picture form to help you guys make the right choices if you choose this "nice to have" mod. Ensure you have the proper oil pick up tube and oil filter adapter to go along with the oil pan of your choice. For example if swapping an H/C aluminum pan onto a 97 Bonneville you want all three items. In a Regal you need the pan and pick up tube only.
W body pan with mount provision. W body with steel subframe requires the removal of the lip on the outter part of the mount bucket. It's a quick cut/grind.
H body pan