I have a 48 .inx 66 .in opening. Can I just cut 6 .in off (72 .in) one end or do I have to remove 3 .in from both ends in order to maintain the proper drain slope?

We generally recommend cutting 3 .in off each side, as that would center the drain if that works for your installation. If your drain location is limited, you may cut the base as needed to so the drain hole in the base lines up with the drain without affecting the slope, which is constant from the edge to the drain hole.